Welcome to our gemology dictionary.
Here you will find a comprehensive list of all the essential concepts and definitions in gemmology. Each term is explained clearly and precisely. These definitions will gradually be supplemented with detailed practical guides to help you better understand the concepts of gemmology.
Absorption bands
Visible under a spectroscope, they are dark vertical bands that leave a gap in the visible spectrum.
Absorption line
A dark, sharp, distinct vertical line in a gemstone’s absorption spectrum. These lines, visible using a spectroscope, result from the gemstone’s absorption of certain wavelengths of light. They are characteristic of each mineral and are caused by electronic transitions in the atoms or ions present in the stone’s crystal structure. They help to identify the nature of a gemstone, and sometimes to determine the element(s) responsible for its colour, its chemical composition, and certain treatments.
Absorption spectrum
Refers to the spectrum produced when white light passes through a substance. Certain wavelengths are absorbed by the atoms or ions present, resulting in dark bands or lines against a bright background. In gemmology, this spectrum is characteristic of a stone’s chemical composition and helps to identify it.
Adularescence
A shimmering reflection beneath the stone’s surface caused by interference at the interfaces between the stone’s layers.
Aggregate
A mass consisting of a combination of crystals of the same or different types.
Allochromatic
Refers to a mineral that is colored by impurities or trace elements in its crystalline structure, but which are not part of the mineral’s inherent chemical composition.
Amorphous
Refers to a substance that does not have a crystalline structure, examples include amber and opal.
Angle of refraction
The angle formed between the refracted ray (the light passing through the surface) and the normal (the line perpendicular to the interface between the two environments) in a refraction phenomenon.
Anisotropic
Refers to a material whose physical properties vary depending on the direction in which they are measured. This directional variation in properties is often related to the material’s crystal structure, which is ordered in a regular but asymmetrical manner in space. In gemmology, anisotropy is primarily observed through characteristic optical phenomena such as birefringence and pleochroism. When measuring with a refractometer, the variation in the refractive index depending on the direction is a criterion for identifying a gem or an anisotropic material. Of the seven crystal systems in the Bravais lattice, only minerals in the cubic system are isotropic, all others are anisotropic.
Anomalous Double Refraction (ADR)
False indication of double refraction caused by internal stresses in a material, which formed during its formation.
Artificial crystal (Homocreate)
Refers to a man-made crystalline substance for which no natural equivalent is known.
Asterism
An optical phenomenon in which a shimmering cross- or star-shaped pattern appears on the surface of a gem.
Aventurescence
An optical effect of shimmering caused by the presence of numerous inclusions, often flat and metallic, that reflect light.
Biaxial
Refers to a material that has two optical axes.
Birefringence
It corresponds to the difference between the highest refractive index and the lowest refractive index of an anisotropic material.
Bleaching
A chemical treatment designed to bleach a stone, often carried out prior to dyeing. Examples of stones commonly treated in this way include jade and pearls.
Brilliance
The amount of light reflected by a gemstone.
Brilliant cut
A cut style characterised by an arrangement of triangular and diamond-shaped facets, arranged radially, which optimises the reflection, sparkle and dispersion of light, the latter factor, however, depends on the optical properties of the material (dispersion plus a possible ‘combo’ or ‘boost’ effect due to birefringence).
Cabochon
A type of gemstone cut, unfaceted, usually convex with a rounded, smooth top and a flat or slightly rounded bottom.
Calibrated stone
Refers to a gemstone cut to precise and standardized dimensions, often used in jewelry to fit perfectly into a setting or a series.
Cameo
A piece of jewellery or ornament carved in relief, often depicting a portrait or a mythological scene, set against a background that is usually of a contrasting colour, and typically made from materials such as shell, coral or glass.
Carat (symbol: ct)
A unit of measurement used to indicate the weight of gemstones, 1 carat = 0.2 grams
Chatoyancy
An optical effect that appears as moving light or a silky reflect, often in the form of a band of light. This phenomenon occurs when light is reflected off fine parallel inclusions (silks) or internal structures (hollow tubes).
Chemical composition
It describes the nature and quantity of the atoms present in a substance.
Chelsea filter
An optical device consisting of a filter glass impregnated with metal oxides that allows certain wavelengths (mainly in the red and green spectrum) to pass through, used to observe how gemstones react to light in order to aid their identification, particularly to detect gemstones containing chromium (Cr).
Chevron growth pattern
This term refers to patterns characteristic of certain hydrothermal synthetic gemstones, resembling the teeth of a saw. Hydrothermal synthetic emeralds exhibit this type of pattern, although it can sometimes be difficult to observe.
Chromium tourmaline
A trade term referring to a vivid green tourmaline coloured by traces of chromium and/or vanadium. When viewed under a Chelsea filter, it may exhibit a characteristic pink to reddish colour, which is due to the chromium.
Chromophore
An atom or group of atoms in a molecule or mineral that absorbs certain wavelengths of light, giving the substance its color. Common chromophores include chromium, iron, copper, and vanadium.
Clarity
Clarity refers to the degree of internal quality of a gemstone. It is assessed based on the quantity and visibility of inclusions and fractures, in relation to the type of gem.
Cleavage
Refers to an internal fracture in a mineral resulting from the cleavability capacity of certain minerals.
Cleavability
The ability of a mineral to break cleanly and regularly along weak atomic planes in its crystal lattice. This property depends on the specific arrangement of atoms in the mineral’s crystal structure, which allows it to break easily along specific directions. In geology and mineralogy, cleavage is an important criterion for identifying minerals.
Color center
A defect in the atomic structure of a material that can absorb certain wavelengths of light and produce a colour.
Color zoning
Refers to areas of different colors within a gem, typically appearing as distinct bands or angular zones. These color zones result from variations in the mineral’s growth conditions (temperature, fluid chemistry, radiation, etc.).
Colored stone
Refers to any gemstone other than a diamond.
Colour change
A phenomenon in which a gemstone changes its colour appearance depending on the type of light source.
Colour range
The range of colours in which a single gem can be found.
Contact metamorphism
Contact metamorphism is the mineralogical and structural transformation of a rock caused by a significant rise in temperature, often due to contact with a magma intrusion ranging from a few meters to several hundred meters in thickness. This transformation generally occurs in the rocks located around the magma, within a limited zone known as the metamorphic aureole, and takes place under relatively low pressure.
Crazing
A network of cracks that can develop in an opal during:
- A reduction in the moisture content of the material. Example: rapid drying using ambient air with low humidity after extraction.
- Exposure to a heat source. Example: halogen lamps in display cases, temperature fluctuations.
- Significant changes in humidity. Example: A change in storage conditions, from dry to humid, and then from humid to dry.
Critical angle
The angle of incidence below which light is completely refracted by the material. In the case of a gemstone, this means that the incident light will not be reflected by the pavilion but will escape through it, resulting in a lack of brilliance.
Crown
The upper part of a faceted gem, above the girdle and below the table. It plays a key role in the gem’s appearance. A well-cut crown enhances the brilliance and scintillation of a faceted gemstone.
Cryptocrystalline
Refers to a substance composed of very small crystals, visible only under very high magnification.
Crystal
Refers to a solid whose atoms, ions, or molecules are arranged in a regular and orderly manner, following a repeating pattern in space. This regular structure influences its physical properties (such as hardness, conductivity, dispersion, etc.).
Crystal habit
The habit of a mineral refers to the general shape, appearance or external configuration of a crystal or aggregate of crystals. This includes size, crystal form, proportions, surface texture, and the way in which the crystals are arranged in relation to one another. The habit of a mineral can provide clues about its crystallisation conditions and the environment in which it formed.
Crystal structure
Refers to the regular and repetitive arrangement of atoms in a crystal, forming an ordered lattice in space. In conjunction with the chemical composition, it determines the physical and optical properties of the mineral, such as its hardness, any cleavage it may exhibit, and its behavior in relation to light.
Crystalline face
Said of a flat surface that delimits a crystal. It can vary in size and shape, and present characteristics such as growth patterns or pits.
Curved growth zone
This term refers to the curved growth lines found in synthetic gemstones produced using the Verneuil (flame fusion) and pulling processes. Curved growth zones are generally less common in synthetic gemstones produced using the pulling process than in those produced using the Verneuil process, due to the differences between these two processes.
Cutting style
It refers to the way in which a gemstone’s facets are arranged to influence its optical effects. There are three main cutting styles: the brilliant cut, characterised by triangular or diamond-shaped facets arranged to maximise the gemstone’s scintillation, the step cut, featuring rectangular or trapezoidal facets arranged in parallel steps, which places less emphasis on scintillation but enhances the stone’s colour whilst producing a more uniform brilliance. Finally, the mixed style combines these two approaches to achieve a balance between scintillation, brilliance and colour enhancement. At the same time, there is a fourth style, less common in the gemstone market, known as the fantasy cutting style, which notably features concave or convex curves.
Dark-field illumination
A microscopy technique in which light illuminates the sample at an oblique angle. This method provides high contrast and helps to observe fine details that would be difficult to see under direct illumination.
Density
The relationship between the mass of a body and its volume. Example: 1 kilogram of water has a volume of 0.001 m³ (at 4°C), which corresponds to a density of 1.
Theoretical densities of some minerals:
Quartz: ~2.65
Calcite: 2.6–2.8
Peridot: ~3.27
Cerussite: 6.58
Spinel: 3.53–4.08
Deposit
Refers to a natural accumulation of minerals or exploitable resources present in the ground and/or the Earth’s subsurface. These materials can be extracted for industrial, artisanal, commercial, or scientific use. The term is commonly used to describe reserves of metals, fossil fuels, or other mineral substances. Deposits can vary in size, composition, and quality, and are often exploited by extraction methods such as drilling, mining, or open-pit or underground quarrying.
Dichroism
This term refers to a pleochroic gemstone that exhibits two colours or shades of colour depending on the direction from which it is viewed or the direction of the polarised light.
Dichroscope
A gemological instrument used to observe two of the colours of a pleochroic gemstone from a given viewing angle.
Diffraction
Particular interference phenomenon by which white light is decomposed into its spectral colors. Diffraction is responsible for the play of colors in opals.
This phenomenon occurs through constructive interference of waves reflected at the interfaces of the regular alternation of microstructures with different refractive indices within a mineral (microfractures, crystal lattices, ordered stackings, etc.).
Diffusion (treatment)
Treatment used to modify or enhance the color of a gemstone by introducing chemical elements at high temperature to a greater or lesser depth beyond the surface of the gemstone (generally 0.5 mm). Some very pale yellow or even colorless sapphires are diffused with beryllium to give them a more desirable orange color.
Warning: as this treatment is generally shallow (~0.5 mm), gemstones that have undergone diffusion cannot be recut, at the risk of a partial or total reduction in saturation, or even the appearance of a different color not initially visible.
Dispersion
A physical phenomenon whereby white light, or any other polychromatic light, is separated into its various spectral colours as it passes through a prism or a refractive material. When a gemstone disperses light sufficiently, it produces coloured flashes known as ‘fire’, which can be observed in diamonds, cerussite, sphalerite, wulfenite and other minerals.
Double refraction
Phenomenon by which the crystal structure splits light into two rays traveling at different speeds inside the crystal, and where each ray follows a slightly different direction.
Doublet
Process consisting of assembling a genuine stone, often of lower quality, with a lower-value material (glass, synthetic stone, quartz, etc.) in order to imitate more expensive stones or to give the stone a particular color or luster. Some opals undergo this process in order to enhance their color and bring out their play of colors.
Durability
The durability of a gemstone refers to its ability to resist wear, impacts, scratches, and deterioration over time. It is based on several physical and chemical properties, such as hardness, toughness, and chemical stability. In summary, it refers to a gemstone’s resistance to everyday conditions (impacts, exposure to air, humidity, temperature variations, chemical products), while preserving its appearance and beauty.
Dyeing treatment
Process consisting of introducing coloring substances into a stone in order to modify or intensify its color. The dye preferentially penetrates cracks, pores, or altered zones of the material, making this treatment particularly effective on porous or microfractured gemstones. It is often preceded by operations such as bleaching, intended to remove or reduce the original color to allow a more uniform recoloring, and/or thermal shock, used to create or enhance a network of microcracks that facilitates the penetration of the dye. Used to reproduce more desirable colors, its stability is variable, as the color may change under the effect of light, heat, or certain chemical agents.
Electrical conductivity
The ability of a substance to conduct an electric current.
Emission line
A vertical line of higher light intensity than the spectral lines (transmitted wavelengths). These lines, which can be observed using a spectroscope, result from fluorescence…. An example is spinel (chromium), but this is inhibited if there is too much iron.
Emission spectrum
Refers to the spectrum produced by a substance that emits light, usually when excited (by heat, other radiation or an electric discharge). It appears as coloured lines of light against a dark background, with each chemical element producing a specific spectrum.
Erosion
Refers to a geological process by which rocks and minerals at the Earth’s surface are worn down, fragmented, and transported by natural agents such as water, wind, ice, or gravity. This phenomenon acts on landscapes over time, modifying their shape and structure. Erosion can be slow and gradual or faster depending on environmental conditions. Several types of erosion mechanisms are distinguished: mechanical, water-based, chemical, wind-based, glacial, etc.
Euhedral
Refers to a well-formed mineral that takes the form of a crystal bounded by distinct crystal faces.
Extinction areas
This term refers, in a faceted transparent gemstone, to areas that appear dark or black when observed from a certain angle. This phenomenon is more or less pronounced depending on the material, the proportions of the stone, and the angles of the facets.
Extrordinary ray
It is a light ray that follows a different trajectory from that of the ordinary ray when it passes through a birefringent crystal. In this case, the speed of light propagation depends on the angle of incidence and the direction of propagation relative to the crystal. The extraordinary ray results from the complex interaction between light and the optical properties of the material, which can lead to a deviation of the light.
Facet
Refers to a flat surface cut on a gemstone, usually to maximise its brilliance and lustre.
Fancy cut
A term whose meaning varies depending on the material:
– For diamonds, it refers to all cuts other than the round brilliant cut.
– For coloured gemstones, it refers to a free-form cut, which may feature flat and/or curved surfaces, without any strict stylistic constraints.
Fancy shape
Includes any cut of gemstone other than the round brilliant cut.
Filling (Treatment)
Fracture filling is a treatment consisting of filling the fissures or fractures of a gemstone with a transparent substance (such as oil, resin, or glass) in order to make them less visible and improve the appearance of the gemstone. The filler must have a refractive index close to that of the gemstone so that light passes through uniformly, thereby making the fracture less noticeable.
Fine color
A gemstone color considered to be among the most desirable in the market.
Fingerprint inclusion
An inclusion that often resembles a fingerprint, frequently associated with the healing of a fracture. Please note that this type of inclusion can be found in both natural gemstones and synthetic stones.
Finish
The quality of the polish and cut of a gem.
Fire
Refers to the coloured flashes produced when a gemstone disperses light.
Flame fusion (Verneuil process)
Main method for producing synthetic corundum crystals, based on the fusion of alumina powder (Al₂O₃) using an oxyhydrogen torch. The molten material (alumina) gradually deposits onto a seed crystal, forming a stalagmite-shaped crystal called a “boule.” The color of the crystal is determined by the addition of metal oxides, such as chromium for red, cobalt for blue, etc. This process is also used for the production of synthetic spinels, as well as artificial crystals such as strontium titanate.
Fluid inclusion
Said of an inclusion consisting of a cavity filled with a liquid.
Fluorescence
Physical phenomenon in which a material emits visible light (or sometimes ultraviolet light) after absorbing energy, generally in the form of ultraviolet light. During this process, the material’s electrons are excited by the absorption of a photon, then re-emit this energy as light of a longer wavelength (or a different color). Fluorescence stops quickly after excitation, unlike phosphorescence, which persists longer.
This phenomenon is used to help identify gemstones when they are exposed to short-wave or long-wave UV light. For example, certain stones such as sapphire, ruby, or amber may exhibit distinct fluorescent colors under UV light, which helps distinguish them from other materials of similar appearance. The color of fluorescence can vary depending on the chemical composition of the material, making it a valuable tool in gemstone identification.
Fracture
Any fracture in a gemstone, with the exception of cleavage, twinning planes, intergrowth and non-twin interpenetration.
Gem
A gemstone is a mineral stone, sometimes of organic origin, which is neither synthetic or artificial, has been cut, and possesses specific qualities such as durability, beauty and rarity. These characteristics give it a special value and justify its preferred use in jewellery or its preservation in a collection. This definition allows gems to be accurately identified and classified, whilst acknowledging that their appeal extends far beyond mere technical criteria.
Gemmology
Gemmology is the multidisciplinary science concerned with the study of gemstones, combining knowledge of mineralogy, geology, chemistry and physics. Its main objective is to identify gemstones, determine any treatments they may have undergone, distinguish them from synthetic, hybrid, counterfeit and imitation gemstones, analyse characteristic inclusions, determine their geographical origin and assess their quality and value.
Geographical origin
Refers to the natural location where a gemstone or mineral was formed and mined, corresponding to the deposit, region, or country of origin, such as Myanmar (formerly Burma) for certain rubies, Sri Lanka for many sapphires, or Colombia for emeralds. This concept allows a stone to be linked to a specific geological context and serves as a basis for scientific study through the analysis of mineralogical and chemical characteristics, as well as inclusions, which can reveal the conditions under which the deposit formed. It also plays an important role in the traceability of gemstones and in regulating the trade in stones, particularly within the framework of international initiatives such as the Kimberley Process for diamonds. Finally, geographic origin can influence a gem’s commercial value when certain sources, renowned for the quality or rarity of their stones, are particularly coveted on the market.
Germinative crystal (crystal seed)
An embryonic crystal that forms naturally or through chemical synthesis. Seed crystals are used as seeds in the growth of certain synthetic crystals.
Girdle
The narrow section of a gemstone between the crown and the pavilion
Goldstone
An imitation of an aventurescent feldspar (sunstone) made from coloured glass to which fragments of a reflective material have been added to reproduce the aventurescence effect (typically copper flakes to imitate sunstone).
Group
A mineral group is a group of minerals that share a common crystal structure and similar chemical composition, but which may differ slightly in terms of specific chemical elements. One example of a group is that of garnets, which comprises two series (pyralspite = aluminium garnets and ougrandite = calcium garnets), each of which in turn comprises various species (almandine, pyrope, ouvarovite, etc.).
Growth by anhydrous dissolution (Flux)
A synthesis process in which mineral salts are dissolved in heated chemicals and then crystallise as they cool.
Hardness
Elle désigne la résistance à la rayure d’un matériau. C’est l’une des propriétés physiques les plus importantes pour évaluer la durabilité d’une gemme. La dureté d’une pierre est mesurée selon l’échelle de Mohs, qui classe les minéraux de 1 à 10, 1 étant la dureté la plus faible (par exemple, le talc) et 10 la plus élevée (le diamant).
Heat treatment
A process involving the exposure of a gemstone to high temperatures in order to enhance its appearance, particularly its colour, clarity or certain optical properties. The results achieved are generally stable and long-lasting.
Heavy liquid
A liquid of known density used in gemmology to estimate the density of a gemstone. The gem is immersed in the liquid, and its behavior (floating, suspended, or sinking) allows one to determine whether its density is lower, approximately equal, or higher than that of the liquid. This method serves as a quick tool to aid in gem identification, complementing more precise measurements such as those obtained using a hydrostatic balance.
Hololith
Refers to a piece of jewellery crafted (carved) from a single piece of rough stone.
Hue
Hue refers to the colour of a gemstone that is immediately apparent, that is to say, the overall chromatic appearance that is evident upon observation. It enables the identification of a gemstone’s dominant colour, regardless of its saturation or tone, and is one of the key criteria for describing colour in gemmology.
Hybrid stone
Although rarely used, this term refers to a stone that combines natural and synthetic materials, including treatments such as filling fractures with glass or polymers, or the creation of doublets and triplets.
Hydrostatic balance
A hydrostatic balance is a measuring instrument used to determine the density (or specific gravity) of a solid by weighing it in air and then in a liquid, usually water. It is based on Archimedes’ principle, which states that a body immersed in a fluid experiences a buoyant force equal to the weight of the fluid displaced. In other words, the density of an object corresponds to the mass of water displaced when it is immersed.
Hydrothermal
In geology, this term refers to the circulation of fluids (hydrothermal fluids) rich in chemical elements and mineral salts in the upper part of the continental crust, and to the associated mineral formations.
In gemmology, this term refers to a process for synthesising crystals. The process involves recreating, within an autoclave (≃pressure cooker), the temperature and pressure conditions found in natural hydrothermal veins. The synthetic crystals produced by this process form from seed crystals immersed in an aqueous solution enriched with chemical elements.
Hydrothermal fluid
A hot, pressurised solution capable of dissolving, transporting and depositing minerals through geological structures at varying depths.
Idiochromatic
Refers to a substance in which the coloring chemical element is an integral part of its chemical composition. En d’autres termes, la couleur d’un minéral idiochromatique est intrinsèquement liée à sa composition chimique, exemple : le cuivre (Cu) de la turquoise (CuAl6(PO4)4(OH)8·4H2O).
This contrasts with allochromatic minerals, whose colour depends on the accidental presence of traces or impurities of chromophore chemical elements.
Igneous rock
An igneous rock, also known as a magmatic rock, is a rock that forms when magma, meaning molten rock inside the Earth, or lava on the surface, cools and solidifies.
There are two main types of igneous rock. Intrusive rocks form deep underground, where cooling is slow, allowing large, visible crystals to form, as in granite. Extrusive rocks, on the other hand, form on the Earth’s surface during volcanic eruptions. Cooling is rapid here, resulting in very small or invisible crystals, as in basalt.
Igneous rocks play a vital role in the formation of the Earth’s crust and are responsible for many geological structures.
Imitation stone
Refers to an artificial, synthetic or natural material used to imitate another stone, often a gemstone of considerable value.
Immersion
A technique used to estimate the refractive index of a transparent gemstone and to determine whether it has undergone diffusion treatment. Immersion involves observing the gem being tested in a refractive liquid that most closely matches its index of refraction.
In cases where the gemstone’s refractive index is unknown, the refractive fluid whose refractive index is closest to that of the gemstone being tested will be the one that most effectively flattens the gemstone’s surface.
When viewed through a suitable IR-refracting liquid, diffracted gems often exhibit greater colour saturation along the edges, or an inner section that is slightly different in colour, or even colourless.
Impregnation
A treatment involving the filling of pores and other openings in a gemstone (often cabochons) with molten wax, resin, a polymer or plastic, to improve its appearance and stability. Turquoise is one of the stones most commonly treated by impregnation.
Inclusion
Refers to a solid, liquid or gas trapped within a mineral.
Indigolite
Terme commercial qui désigne une tourmaline bleue.
Inorganic
Refers to a material that is not of biological origin
Intaglio
Refers to gemstones, often flat, that have been engraved with recessed designs.
Interference
Refers to the interaction between two or more coherent waves of the same frequency.
Intrusion
Refers to a body of igneous rock that has crystallised at varying depths.
Iridescence
A phenomenon that causes colours to shift on certain surfaces depending on the angle of view, as with oil floating on water, due to the way light interacts with thin films.
In other words, some light waves are reflected by the upper surface of the film, whilst others penetrate and may in turn be reflected by the lower interface with the surrounding medium, where they can interfere constructively with the earlier waves. Depending on the refractive index, the thickness of the film, and the angle of incidence of the light rays, some reflected waves remain in phase (add together with others = constructive interference), whilst others undergo partial or total phase shift (total = destructive interference).
Irisation
Similar to iridescence, but referring more specifically to colour variations caused by variations in the thickness of a microstructure, such as a crack within a gemstone.
Irradiation
A treatment involving the exposure of a gemstone to electromagnetic radiation (X-rays, gamma rays) or subatomic particles (electrons, neutrons) in order to alter or intensify its colour. Irradiation causes changes in the crystal’s electronic structure, creating or accentuating colour centers. The treatment may be permanent or may require additional heating to stabilise the colour. Sometimes, it is not possible to stabilise the colour of an irradiated stone. When regularly exposed to light or a heat source, the gemstone is then destined to gradually lose its saturation or revert to its original colour. Topazes with long-lasting commercial hues such as Sky Blue, Swiss Blue or London Blue are often irradiated and then heated to stabilise the colour.
Isochromes
Although sometimes difficult to observe, isochromes appear as concentric coloured rings when a material is analysed using a polariscope and conoscope. They are in fact interference fringes that are concentrically distorted by the conoscope. Together with isogyric fringes, they form an optical figure that makes it possible to determine whether an anisotropic gemstone is biaxial or uniaxial.
Isogyres
Refers to the dark areas observed in the optical patterns of anisotropic gemstones, resulting from the extinction of light polarised in certain optical directions.
Isometric
Generally speaking, « isometric » means « of equal measure » and applies to distances, dimensions or proportions, depending on the field. In crystallography, a crystal is said to be isometric when its three axes are approximately equal. Isometric crystals belong to the cubic crystal system.
Isomorphic substitution
Isomorphic substitution is a mineralogical phenomenon whereby one ion is replaced by another of similar size and charge within the crystal structure of a mineral, without altering its overall organisation. This replacement, made possible by the similarity of ionic radius and charge, allows the structure to be preserved whilst varying the chemical composition.
This substitution can result in variations in properties such as colour, density or hardness, and lead to the formation of continuous series between different chemical poles. For example, within the garnet group, we distinguish the pyralspite series (pyrope, almandine, spessartine) and the ugandite series (grossular, andradite, uvarovite), where different cations are partially or completely substituted whilst keeping the same crystal structure.
Isotropic
This term refers to a material, a gemstone whose optical properties are identical in all directions. An isotropic gemstone has a single refractive index and is therefore not birefringent.
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Labradorescence
An optical phenomenon characteristic of labradorite, in which coloured flashes appear on the mineral’s surface. This phenomenon results from the diffraction of light by alternating layers of feldspars with different chemical compositions (potassium and calcium-sodium). These layers form during the sub-solidus exsolution of the feldspars as the rock cools slowly. The lamellar structure creates light interference, thereby producing distinct flashes of colour.
Light
Light is an electromagnetic wave that propagates in a vacuum at a speed of c=299,792,458 m/s. It is characterized by a wavelength and a frequency. The part of light that is visible to the human eye is called visible spectrum (from 380 nm to 780 nm). In physics, light can also be considered as a flow of photons (energy particles), which illustrates its dual wave-particle nature. It helps explain the visual appearance of gemstones and plays an essential role in their identification.
Luster
It refers to the ratio of reflected light to incident light. It describes the way a mineral reflects light, as perceived by the eye. Luster is also what gives gemstones their brilliance.
There are several types of lustre: metallic, submetallic, vitreous, adamantine, silky, pearly, greasy, waxy and earthy.
As a general rule, brilliance is linked to the refractive index and is largely independent of colour:
vitreous lustre: refractive index 1.3 to 1.9
adamantine lustre: refractive index 1.9 to 2.6
submetallic lustre: refractive index 2.6 to 3
metallic lustre: refractive index greater than 3
Macroscopic
Refers to what is visible or measurable on a human scale, without a microscope, as opposed to microscopic. In other words, it is what can be observed directly with the naked eye without a magnifying instrument.
Matrix
It refers to the matrix or support on which crystallization occurs.
Metamict
Refers to a mineral whose crystal lattice is partially or completely disrupted by internal radiation caused by the presence of radioactive elements (uranium, thorium), and which may evolve into an amorphous state while retaining its chemical composition.
Metamorphic rock
A metamorphic rock is a rock that results from the transformation of a pre-existing rock, known as the parent rock, under the influence of high pressure and high temperatures, without passing through a liquid state.
This process, known as metamorphism, alters the structure, texture and sometimes the mineralogical composition of the rock. Minerals may rearrange themselves to form new, more stable assemblages under the new conditions.
Metamorphic rocks generally form deep within the Earth’s crust, particularly during mountain formation or near magma masses. For example, marble is formed from the transformation of limestone, and schist results from the metamorphism of clay-rich rocks.
Microcrystalline
Refers to a rock or texture composed of very small crystals, generally invisible to the naked eye and observable only under a microscope.
Milk and Honey Effect
An optical phenomenon observed in certain chatoyant gemstones when held perpendicular to a light source, particularly chrysoberyls. This results in alternating opaque and translucent areas, resembling a mixture of milk and honey.
Milky
In geology, a mineral is described as milky when it has a translucent appearance with light scattering within it, giving it a cloudy or milky appearance. This opacity is often due to the presence of inclusions or microstructures that prevent light from passing clearly through the material, creating a sort of internal haze.
Mineral
A solid inorganic substance that occurs in the form of a crystal or a crystalline solid.
Mineraloid
A mineraloid is a mineral-like substance that resembles a mineral but lacks a defined crystalline structure, unlike minerals, which have an ordered crystal lattice. Mineraloids are amorphous and often form through rapid cooling or chemical precipitation.
Common examples of mineraloids include amber and opal.
Mixed cut
A cutting style that combines the brilliant cut with the step cut, typically featuring a brilliant-cut crown and a step-cut pavilion, which balances sparkle, brilliance and colour enhancement.
Monochromatic
Refers to light consisting of a single wavelength or a very narrow range of wavelengths. This means that, for a monochromatic light source, all light waves have the same frequency and energy, corresponding to a single spectral color.
A well-known example is the red laser used in laser pointers, whose light is nearly monochromatic and has a wavelength of approximately 650 nm.
Needle
A long, thin, solid, or tubular inclusion that can contain gas and/or a fluid.
Negative crystal
A cavity within a crystal that has the same crystal structure as the host crystal. The cavity resembles an included crystal and may contain solid, liquid and/or gaseous inclusions.
Nucleation
This term refers to the initial stage of a synthesis process in which a crystal begins to form in a supersaturated solution as it cools, when the conditions of temperature, pressure, and concentration allow particles to spontaneously organize into a stable crystal nucleus. This synthesis method eliminates the need for seed crystals in certain syntheses.
Opalescence
Refers to the optical property of a transparent or translucent gemstone that, due to the phenomenon of diffusion, gives it a milky appearance or hue.
Optical axis
A uniaxial refractive axis of an anisotropic material.
Optical character
Refers to the way in which a mineral interacts with light (unirefringent, negative or positive uniaxial birefringent, negative or positive biaxial birefringent)
Optical doubling
A visual effect in which a facet, a relief or an image within the gem appears to be duplicated.
It occurs in birefringent (anisotropic) gemstones, where light splits into two rays travelling at different speeds within the crystal. Each ray follows a slightly different path, producing two slightly offset images as seen by the observer. Corundums (sapphire, ruby) and peridots are known to exhibit optical double refraction. Generally speaking, the higher a gemstone’s birefringence, the more pronounced the optical double refraction effect.
Optical figure
A luminous pattern that can be observed when examining a gemstone using a conoscope and a polariscope. This pattern provides information about the gemstone’s optical properties, making it possible to determine, in the case of an anisotropic gemstone, whether it has a biaxial or uniaxial structure.
Optical sign
The optical sign is a property of birefringent crystals that characterises the relationship between the refractive indices and the propagation of light within the stone. In a uniaxial crystal, the sign is said to be positive when the extraordinary index is greater than the ordinary index, and negative in the opposite case. In a biaxial crystal, the sign is positive when the Z-axis corresponds to the highest refractive index, and negative when it corresponds to the lowest index. The optical sign can be determined by measuring the refractive indices with a refractometer or by observation under a polariscope using a quartz wedge or other compensating plates, which allow the direction of birefringence to be identified. The optical sign is thus a useful criterion for identifying a mineral species and characterising a gemstone.
OTL (Over The Limit)
In some cases, particularly when a gem’s refractive index is too high, generally above 1.79, it is no longer possible to measure the refractive index using a conventional refractometer. This is referred to as “OTL,” which stands for “Over The Limit.” Under these conditions, it is possible to measure the refractive index of an “OTL” gem using a digital refractometer, or by calculating the actual and apparent depths.
Ordinary ray
A light ray that propagates through a birefringent crystal along a path influenced by the crystal’s structure. Its speed of propagation and refractive index (nₑ) vary depending on the direction within the crystal. It splits into two rays and may change direction depending on the crystal’s orientation.
Organic
Refers to materials that come from living organisms or have a biological origin. These materials originate either from the animal kingdom or the plant kingdom and are distinct from mineral materials, which are formed solely by geological processes without any biological influence. Among the best-known examples of organic gemstones are pearls, which form inside mollusks in response to the intrusion of a foreign body, and coral, which comes from colonies of marine corals. Amber is also a gem of organic origin, being fossilized tree resin that sometimes preserves inclusions of organisms (for example, insects).
Oxidising environment
Refers to an environment in which oxidation reactions can occur, meaning that atoms or molecules can lose electrons, usually in the presence of oxygen or other oxidising agents. This type of environment is conducive to chemical processes in which substances react with oxygen, which can lead to changes in the chemical and physical properties of materials.
Padparadscha
A trade name referring to a rare, orange-pink sapphire color.
Paraíba tourmaline
A trade term referring to a tourmaline with vivid neon blue to green colours, caused by the presence of copper (Cu²⁺), originally discovered in the state of Paraíba in Brazil
Parallax
This refers to a fluctuating or shifted reading of the refractive index (RI) on a refractometer. This phenomenon, which can lead to incorrect gem identification, may be caused by an excess of liquid, an irregular surface, or a slight movement of the head while reading the RI on the refractometer.
Pavillon
The lower part of a faceted gemstone, located beneath the table and the girdle. The pavilion allows light to be reflected within the gemstone and emerge through the crown. A properly proportioned pavilion maximizes the brilliance of a faceted gemstone and influences its color saturation.
Phosphorescence
Refers to the phenomenon of luminescence, in which a material continues to emit light after the excitation source (often UV or X-rays) has been turned off. This process results from the absorption of photons that excite electrons to an excited level, followed by a transition to the ground state with a time delay, thereby releasing energy in the form of light.
Piezoelectricity
Piezoelectricity is the ability of certain materials to generate an electric potential difference (a change in electric charge between two points) when subjected to mechanical pressure or deformation. This phenomenon occurs particularly in crystals such as quartz, where the electric charges within the crystal move in response to the applied stress, thereby generating an electric current.
Play of colour
The visual effect of precious opals, characterised by flashes of iridescent colour visible on the surface or within the stone when the viewing angle or lighting changes.
The play of colour is caused by the diffraction of light, brought about by the opal’s highly ordered internal structure, which consists of microscopic spheres of hydrated silica arranged in a regular lattice. When light passes through this structure, certain wavelengths are reflected whilst others are transmitted, some ‘add together’ (constructive interference), producing these spectacular colours.
Pleochroism
A property of anisotropic minerals to exhibit different colors depending on the direction of observation, due to selective absorption of light. There are two forms of pleochroism: dichroism (the presence of two different colors) and trichroism (the presence of three different colors).
Pneumatolytic
Refers to a geological process that occurs during the final stages of magma crystallization, characterized by the action of fluids rich in steam, gases, and chemical elements high in metals and halogens (boron, chlorine, beryllium, fluorine, lithium, niobium, tin) that migrate toward fractures or cavities in the granite and deposit specific minerals.
These pneumatolytic minerals include, for example, beryl, tourmaline, topaz, or cassiterite, and often form in pegmatites, greisens, or veins peripheral to the granite. This process differs from hydrothermal phenomena due to its higher temperature (374–600°C) and its predominantly gaseous composition, before water becomes the dominant component during the hydrothermal stage.
Polariscope
A polariscope is an optical instrument consisting of two polarizing filters that allows one to study a gemstone’s reaction to polarized light. When placed between these filters and observed as it rotates, an anisotropic gem exhibits alternating light and dark phases, brightening and then darkening during rotation due to the birefringence caused by its crystalline structure, whereas an isotropic gem remains consistently dark in all positions. This device thus allows for the rapid distinction between these two categories of stones and reveals internal stresses or optical anomalies (such as ADR). When supplemented with a conoscope, it enables the observation of interference figures that distinguish uniaxial gems from biaxial gems, which is useful for identification.
Polarising filter
This term refers to an optical device that blocks certain directions of vibration in polychromatic light. White light, which is generally composed of waves vibrating in all directions, can be polarised, meaning that the waves vibrate in a single, specific direction. A polarising filter allows only light whose vibrations are oriented in a particular direction to pass through, whilst blocking those with other orientations.
Polarization
Light polarization is the phenomenon in which the vibrations of a light wave’s electric field align in a specific direction. In unpolarized light, these vibrations are random, whereas in polarized light, they follow a regular pattern. This phenomenon occurs, for example, during reflection, diffusion, or when light passes through polarizing filters.
Polychromatic
Refers to a light source whose emission spectrum consists of multiple colors. For example, the light emitted by the sun is polychromatic. When we observe the colors resulting from its dispersion (diffraction) through a rainbow, we see that daylight is composed of multiple colors (in reality, a range of wavelengths).
Polychrome
Refers to a stone that displays multiple colors visible to the naked eye, either in distinct zones or as gradients or shades within the crystal. These color variations may be due to the stone’s chemical composition or impurities
Price per carat
It refers to the cost of a gemstone per unit of mass known as a carat, which is equivalent to 0.20 grams. It allows for the comparison of the value of gemstones of the same type (or sometimes different types) regardless of their total size, while taking into account criteria such as color, clarity, weight, variety, and geographical origin, all of which significantly influence the price.
Primary deposit
The concentration of minerals or metals in the rock in which they were formed, without any transport or redistribution.
Provenance
Refers to the provenance or history of ownership of a gemstone or piece of jewelry.
Quench cracking
A treatment that involves subjecting a gemstone to thermal shock in order to create micro-cracks. This treatment is performed prior to dyeing to allow the dye to penetrate into the micro-cracks previously created in the gemstone.
Raman spectrometry
Raman spectroscopy is a non-destructive analytical technique that identifies a substance by measuring the frequency shift of scattered light following interaction with a laser. This shift, known as the Raman effect, is characteristic of the vibrations of chemical bonds and serves as a signature for identifying materials, particularly in mineralogy.
Reducing environment
Refers to an environment in which reduction reactions take place, meaning that atoms or molecules gain electrons. In such an environment, reducing agents (atoms, ions or molecules capable of donating electrons) prevent or limit oxidation reactions.
Reflected light
A lighting technique in microscopy in which light is directed onto the specimen so that it is reflected off the surface of the object, rather than transmitted through it. This technique allows the surface details of gemstones to be observed.
Refraction
Refraction is the change in the speed and direction of light as it passes through the boundary between two materials. In gemmology, it is measured by a stone’s refractive index (RI), and some stones, such as the diamond, exhibit single refraction, whilst others, such as zircon, exhibit double refraction, where light splits into two distinct rays.
Refraction liquid
A liquid with a precise and known refractive index. These liquids are used in gemology in various ways to measure or estimate the refractive index of a gemstone.
High-index liquids (typically ranging from 1.79 to 1.81) are used, in particular, with a refractometer to measure the refractive index or indices of gemstones.
They are also used in immersion methods, where the gem’s refractive index is compared to that of the liquid: when the indices are close, the visual boundary between the gem and the liquid becomes difficult to distinguish, thereby providing an estimate of the gem’s refractive index.
Refractive index (RI)
A physical quantity that describes the behavior of light in a medium. The refractive index (RI) of a material is calculated as the ratio of the speed of light in a vacuum (c = 299,792,458 m/s) to the speed of light in the material in question. The refractive index of most gemstones can be measured using a standard (analog) refractometer.
Regional metamorphism
Regional metamorphism is the large-scale transformation of rocks under the influence of high pressure and high temperatures, generally deep within the Earth’s crust during tectonic movements and the formation of mountain ranges. It can affect very large volumes of rock, extending several kilometers in thickness and covering vast areas. The intensity of metamorphism varies depending on pressure and temperature conditions and is characterized by the formation of new minerals known as neominerals. These transformations occur without any overall change in the chemical composition of the protolith.
Rock
A rock is a natural material that is almost always solid and consists, either entirely or mainly, of a collection of minerals. Rocks can be classified into three main categories according to their origin: igneous (formed by the solidification of magma), sedimentary (formed by the accumulation of sediments, and sometimes containing fossils), and metamorphic (formed by the transformation of pre-existing rocks under the effect of heat and pressure).
Rock cycle
A series of continuous geological processes through which rocks are formed, transformed and recycled from older rocks. This cycle is driven by the nuclear energy released by the Earth’s core, which is the source of the Earth’s geodynamics.
Rubellite
A gemmological/commercial name for a pink to deep red elbaite, belonging to the tourmaline group.
Saturation
This refers to the intensity of a gemstone’s colour. It indicates how vivid the hue is in relation to grey (for cool-coloured gemstones) or brown (for warm-coloured gemstones). Saturation is generally rated on a scale of 1 (low) to 6 (vivid), and varies depending on the mineral species: certain rare gemstones, such as ruby or Paraíba tourmaline, can reach the maximum level.
Secondary deposit (placer)
A concentration of minerals or metals formed following the weathering or erosion of a primary deposit, through the transport and accumulation of usable elements in a new location. Example: alluvial gold deposits are secondary gold deposits formed by the leaching of primary auriferous rocks.
Sedimentary rock
A sedimentary rock is a rock formed by the accumulation, deposition and transformation of sediments, that is, particles resulting from the erosion of other rocks or the remains of living organisms.
These sediments are generally deposited in successive layers in sedimentary basins (at the bottom of seas, lakes or on continents… ). Over time, they compact and cement together to form a solid rock, a process known as diagenesis.
Sedimentary rocks may contain fossils, making them valuable records of the history of life and ancient environments. For example, limestone from an ancient marine environment is often formed from shells, microalgae, skeletons, etc., whilst sandstone results from the accumulation of grains of sand.
Sedimentary rocks cover a large part of the Earth’s surface and play an important role in understanding landscapes and the geological history.
Selective absorption
When an object absorbs certain components of light and transmits others.
Shape
The outline of a cut stone as seen from the front, often defined by its edge.
Silicate
A silicate is a mineral whose structure is based on the fundamental unit known as the silicate tetrahedron, consisting of a silicon atom surrounded by four oxygen atoms, with the formula SiO₄.
These tetrahedra can combine in various ways by sharing certain oxygen atoms, resulting in a wide variety of structures and, consequently, minerals. Depending on their arrangement, several major families of silicates can be distinguished, such as isolated tetrahedral nesosilicates, chain-like inosilicates, sheet-like phyllosilicates, and three-dimensional network-like tectosilicates.
Silicates make up the vast majority of minerals in the Earth’s crust and include many important gemstones such as quartz, feldspars, garnets and micas.
Silk
In gemmology, ‘silk’ refers to a type of inclusion consisting of very fine mineral needles, usually aligned in parallel within a gemstone. These needles are most often made of rutile, but may also consist of other minerals depending on the stone.
These inclusions give the gem a soft, silky appearance, hence their name. They can diffuse light and produce distinctive optical effects. For example, when arranged in multiple directions, they can cause a phenomenon known as asterism, meaning the appearance of a luminous star on the stone’s surface, as seen in certain rubies or sapphires.
The presence of silk is significant in gemmology as it can help identify a stone’s natural origin, distinguish a natural gem from a synthetic one, and assess any heat treatments, as silk can be altered or dissolved by heat.
Simple refraction
Single refraction occurs when the crystalline structure of a gemstone alters incoming light solely through refraction and absorption, without splitting the light into multiple rays. This occurs in crystals with a symmetrical atomic structure, such as diamond, where light is simply refracted without being separated.
Slocum stone
An opal imitation developed by John L. Slocum in the 1970s, consisting of glass containing fine inclusions or interference films of aluminum oxide that produce iridescence similar to the play of colors in natural opal, but not through diffraction. In other words, these interference films act somewhat like very thin layers that create colored reflections depending on the viewing angle.
Smoke treatment
A treatment applied to certain opals involving wrapping the stone in paper (newspaper, brown paper or aluminium foil) or soaking it in oil, the whole is then heated (burned) so that smoke or carbon residues from the combustion penetrate slightly beneath its surface. This carbon infiltration has the effect of darkening the opal’s background colour and bringing out its play of colours more clearly. The alteration remains superficial, however, and often results in an irregular dark appearance.
Sparkle
This refers to the play of light that can be seen when the stone is in movement, produced by the reflections from the facets and giving the impression of moving points of light.
Species
A class comprising minerals that are identical in terms of chemical composition and crystal structure.
Specific gravity (relative density)
The ratio of a substance’s density to that of water. In other words, it is a number that indicates how many times denser or less dense a substance is than water.
Spectrophotometer
A measuring instrument that analyses the light transmitted by a substance as a function of wavelength. It produces an absorption spectrum and identifies the elements or compounds present.
Spectroscope
An optical instrument that allows direct observation of the spectrum of light emitted, transmitted or absorbed by a substance. It decomposes light into its different wavelengths, sometimes revealing characteristic lines or bands. Used in gemmology, it helps to identify a gemstone by comparing the observed absorption spectrum with known reference spectrums.
Spectrum
A spectrum is the distribution of electromagnetic radiation according to its wavelength or frequency. It corresponds to the entire range of light radiation that a source can emit, transmit or absorb, and can be observed using a spectroscope.
Spot reading
A method for measuring the refractive index of a gemstone on a curved surface (typically a cabochon) using a refractometer.
Stability
The ability of a stone to withstand the effects of light, heat and chemicals without any change in its colour, structure or appearance.
Step cut
A cutting style characterised by elongated, rectangular or trapezoidal facets, arranged in parallel rows and stacked in tiers around the faceted stone. This style emphasises the colour of the stone rather than its sparkle.
Subduction
Subduction is a geodynamic process occurring in convergence zones where a tectonic plate, usually an oceanic plate, which is denser, plunges beneath another plate into the mantle. This subduction takes place along an inclined plane (the Wadati–Benioff plane) characterised by high seismic activity. At depth, the subducted plate undergoes transformations and releases fluids that cause magma to form, leading to often explosive volcanism. Subduction also leads to the formation of oceanic trenches and mountain ranges, and contributes to the recycling of the lithosphere into the Earth’s mantle.
Sugar treatment
A treatment for opals that involves soaking them in a hot sugar solution and then immersing them in sulphuric acid. Under the effect of the acid, the sugar is converted into carbon, which settles in the pores of the stone, darkening its background colour and enhancing its play of colour. This treatment affects only the surface layers of the opal.
Surface blemish
Refers to an irregularity confined to the surface of a gemstone.
Synthetic Boule
A cylindrical synthetic crystal, mainly produced using the flame fusion method.
Synthetic stone
Refers to a laboratory-grown gemstone that has essentially the same chemical composition, crystal structure and properties as its natural counterpart.
Table
The main top facet of a faceted gemstone, usually cut parallel to the girdle. However, some fancy-cut gemstones may not have a table, the table may also be inclined to create specific optical effects.
Tabular
This term refers to a crystal or mineral with a flattened habit, characterised by predominant growth in two crystallographic directions and limited growth in the third, giving it a blade-like or plate-like appearance. Baryte, wulfenite and cerussite are examples of minerals that can exhibit this type of habit.
Tenacity
A property that describes a gemstone’s resistance to impact, breakage or shattering. It is distinct from hardness, which measures only resistance to scratching. For example, a material may be hard but not very tough: such as a wine glass, which, although hard, breaks easily when dropped from a height of one metre, whereas a piece of wood, which is less hard, withstands the impact and remains intact due to its greater toughness.
Tenebrescence (reversible photochromism)
A phenomenon observed in certain minerals, characterised by a change in colour when exposed to light, particularly ultraviolet light. This change is reversible, the mineral gradually returns to its original colour once placed back in the dark or, in some cases, when subjected to gentle heat. It is explained by the rearrangement of electrons linked to crystal defects, which alter the way light is absorbed. A classic example is hackmanite, which turns purple under UV light before returning to a pale colour in the dark. Unlike fluorescence and phosphorescence, this is not the emission of light but simply a change in colour.
Thermal conductivity
The ability of a substance to conduct heat through itself.
Thermal shock
Damage caused by a sudden change in temperature.
Three-phase inclusion
Refers to a cavity-like inclusion containing a liquid, a gas and one or more solids.
Tone
Tone refers to the degree of lightness or darkness of a gemstone, regardless of its colour.
Trace elements
Refers to the accidental or systematic presence of chemical elements in extremely small quantities within a mineral (at the parts-per-million (ppm) or even parts-per-billion (ppb) level). Although these elements constitute only a small fraction of a mineral, their presence can have a significant impact on its properties (colour, fluorescence) and its overall appearance.
Transfer of charge
A phenomenon in which electrons move between neighboring ions within a gemstone’s crystal structure, causing selective absorption of light and thereby influencing its color.
Transition elements
Transition elements are metals located in the centre of the periodic table (groups 3 to 12) which, when present in the crystal structure, influence the colour and optical properties of a crystal. These elements, such as chromium, iron or titanium, absorb certain wavelengths of light due to their d-orbitals.
Translucent
Describes a material that allows light to pass through diffusely, without allowing one to see clearly through it.
Transparent
Refers to a material that allows light to pass through without significant light diffusion, making it possible to see clearly through it.
Treatment
Refers to any human intervention, other than traditional gem-cutting practices, aimed at altering the appearance or properties of a gemstone in order to improve its appearance, durability or commercial value.
Triplet
A composite stone consisting of three distinct layers of material, either bonded or fused together, or two layers combined with a layer of colored cement, forming a single stone. Triplets are used to imitate natural gemstones or to enhance color, appearance, and durability, particularly when the individual materials would be insufficient or too fragile to be used on their own.
Triplet magnifying glass
An optical magnifying glass consisting of three lenses that are typically cemented or assembled together, designed to correct the main aberrations (chromatic and spherical), resulting in a very sharp image.
Tweezers
Pliers used to pick up and handle gemstones.
Two-phase inclusion
Refers to a cavity-like inclusion containing a liquid and a gas, or, more rarely, a liquid and a solid, or a gas and a solid.
Two-tone
This term refers to a gemstone displaying two colours in distinct zoning patterns.
Twinning
An ordered arrangement of two or more crystals of the same species, called individuals, joined together according to a specific symmetry rule (plane, axis, or center). A twinning can be simple (two crystals) or multiple (several crystals associated according to the same rule, such as the polysynthetic twinning of plagioclases). It forms by accretion (contact along a face) or by interpenetration. Twinning is often recognized by the presence of reentrant angles or parallel bands.
Twinning plane
Refers to the crystallographic plane that defines the orientation relationship between two or more twinned crystals, associated with specific symmetry operations.
Ultraviolet radiation
Refers to electromagnetic radiation with a wavelength shorter than that of visible light, generally between 10 and 400 nm. In gemmology, UV light is used to detect characteristics of gemstones such as fluorescence, with standard wavelengths of 365 nm (UV-L) and 254 nm (UV-C), which reveal various reactions, such as the blue fluorescence of diamonds or the tenebrescence of hackmanite.
Uniaxial
Refers to a crystal that has a single optical axis, that is, a direction in which light does not undergo birefringence. Les minéraux uniaxes appartiennent aux systèmes cristallins quadratique, hexagonal et rhomboédrique.
In other words, if the refractive index of a perfectly uniaxial anisotropic mineral is measured along its optical axis using a refractometer, it is uniaxial in that direction: the value remains constant, unlike in other directions that do not lie along the optical axis, in which the mineral exhibits birefringence.
Unit cell
It is the smallest unit of a crystal that, when repeated in all three spatial directions, reconstructs the entire lattice and preserves its structure as well as its physical and chemical properties. It can be either primitive or conventional and is one of the 14 Bravais lattices grouped into 7 crystal systems.
Variety
A variety is a form of a mineral species that is distinguished by its color, shape, or distinctive appearance, which gives it a specific name without changing the underlying mineral species. For example, emerald is a green variety of beryl.
Unlike a new species, a variety does not alter the essential chemical composition or the crystal structure. Its differences often stem from trace amounts of chromophore elements or minor variations in chemical composition. More rarely, it is the morphology of the crystals that justifies the distinction, as is the case with certain unusual forms of known minerals.
Vein
A vein is a band of material formed by the intrusion of hydrothermal, magmatic or sedimentary material (often quartz, metal or other minerals or rocks) into pre-existing rock.
Visible spectrum
It corresponds to the part of the electromagnetic spectrum that is visible to the human eye, ranging approximately from 400 to 700 nanometres. It extends from violet to red and constitutes the portion of light that can be observed directly, particularly when white light is dispersed.
Watermelon tourmaline
A trade term referring to a tourmaline, usually an elbaite, exhibiting zoning with a pink centre surrounded by a green overgrowth.
Wavelength
Wavelength (denoted by λ) is the distance between two successive points on a periodic wave that oscillates in the same manner. It is used to characterize a wave and depends on its frequency and speed of propagation. Light is a transverse electromagnetic wave that propagates in a vacuum at a finite speed of approximately 300,000 km/s.
Example for light: visible light has wavelengths ranging from about 400 nm for violet to 700 nm for red, which explains its different colors and its interactions with matter, such as absorption, reflection, or diffraction.
Windowing
A phenomenon that occurs when one can see through the stone by looking at it from the table, making the stone appear visually shallower and often less brilliant. The table then appears dull (lacking brilliance), whilst the outline of the crown may remain brilliant. This defect is linked to several factors, such as the proportion of the pavilion, the arrangement of the facets, the refractive index of the material, and the chosen cut.
Xenocrystal
A xenocrystal is a crystal found in an igneous rock that did not form from the magma itself, but was carried in from a pre-existing rock during intrusion or crystallization.
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