Gemological glossary: every technical term defined

Natural jadeite that has received no treatment beyond cutting and polishing. The highest commercial classification for jadeite, confirming no bleaching, polymer impregnation, or dyeing. Certified by GIA and major laboratories via FTIR spectroscopy.

The floating, billowing light phenomenon seen in moonstone. Produced by thin-film interference between alternating layers of orthoclase and albite feldspar within the stone. The light appears to move as the stone is rotated. Finest adularescence is blue-white and centred; it is the primary quality criterion for moonstone.

A major US gem laboratory founded in 1977, based in New York. Known for the most detailed emerald treatment extent grading (minor/moderate/significant/extensive filling) and for Prestige Reports used at Christie's and Sotheby's auction submissions. Accepted at all major international auction houses.

See: colour change. The specific colour change of alexandrite (green in daylight, red in incandescent light) is sometimes used as a benchmark for other colour-change gems. The term "alexandrite effect" refers to any strong colour change resembling this pattern.

A secondary gem deposit formed when gems are transported by water from their primary geological formation and concentrated in riverbeds, floodplains, or marine sediments. Alluvial deposits produce most of the world's gem sapphires (Sri Lanka, Burma), rubies (Burma), and diamonds (historically India, currently parts of Africa). Alluvial gems are typically water-worn and rounded.

The most common garnet species; an iron-aluminium nesosilicate (Fe3Al2(SiO4)3). Produces dark red to brownish-red colour. The "standard" red garnet of commercial jewellery. Found globally. Mohs hardness approximately 7.5.

Having no crystalline structure; lacking long-range atomic order. Opal and amber are amorphous rather than crystalline minerals. Amorphous materials do not show cleavage and have no definite melting point.

A calcium-iron garnet species (Ca3Fe2(SiO4)3). Gem varieties include demantoid (green, coloured by chromium, with dispersion exceeding diamond) and topazolite (yellow). The rarest commercially significant garnet species at investment quality.

A crystalline form of calcium carbonate (CaCO3). The specific polymorph used by molluscs to build shells and pearl nacre, deposited in flat hexagonal platelets approximately 0.4 micrometres thick. Distinguished from the other common CaCO3 polymorph, calcite, by its orthorhombic crystal structure.

The star optical phenomenon produced in certain gems when light reflects from parallel needle-like inclusions arranged in two or more sets at specific angles. In ruby and sapphire, three sets of rutile needles (silk) at 120 degrees produce a six-rayed star. Twelve-rayed stars occur when two different inclusion orientations are present. Asterism requires cabochon cutting to display.

An internal sparkling or glittering optical effect produced by numerous reflective platelets of mica (in aventurine quartz), hematite, or other minerals distributed through the stone. Distinguished from schiller (which is a play of colours) by being a sparkle effect rather than a colour shift.

Jadeite that has been bleached with acid to remove iron staining and then impregnated with a colourless polymer resin to fill the micro-fractures left by bleaching. Commercially legitimate when disclosed but degrades over years. Detected by FTIR spectroscopy. Severe commercial discount vs A jade.

Natural pearl from the Persian Gulf, named for the Iraqi trading city through which Gulf pearls were historically traded. The benchmark for natural pearl quality; the specific water conditions of the Gulf produced a nacre quality recognised by experienced traders. Gulf pearl fisheries collapsed in the 1930s.

A heat treatment process discovered in commercial production around 1999-2002 in which sapphires are heated in the presence of beryllium, causing beryllium ions to diffuse into the stone's surface and change its colour. Can produce orange "padparadscha-like" sapphires from lower-value material. Detected by LA-ICP-MS. Must be disclosed; not industry standard.

The property of doubly refractive (anisotropic) gems to split a beam of light into two rays that travel at different speeds. The numerical value is the difference between the highest and lowest refractive indices. High birefringence in peridot (0.036) causes visible doubling of back facets when viewed through the table. Singly refractive gems (garnet, spinel, diamond) have zero birefringence.

The fee charged by an auction house to the buyer, calculated as a percentage of the hammer price. At Christie's and Sotheby's Geneva, currently approximately 25-30% of hammer price. The total buyer cost is hammer price plus buyer's premium. Published auction results typically show hammer prices, not total buyer cost.

Jadeite that has been dyed to improve or change colour. May be A jade or B jade in addition to being dyed. The dye fades with UV exposure. Detected by spectroscopy. Must be disclosed. Severe commercial discount.

A gem cut with a smooth, domed upper surface (the cabochon) and a flat or slightly domed base, without facets. Preferred for gems displaying optical phenomena requiring a smooth curved surface: asterism (star), chatoyancy (cat's eye), adularescence (moonstone), and play-of-colour (opal). Also traditional for jade, turquoise, and coral.

The unit of gem weight. One carat equals 0.2 grams exactly. Standardised internationally since 1907. Subdivided into 100 points (0.01 ct = 1 point). Not to be confused with karat (kt or k), which measures gold purity. The word derives from the carob seed (Greek: keration), historically used as a counterweight.

See: chatoyancy. When the term "cat's eye" is used without qualification (e.g., "a fine cat's eye"), it conventionally refers specifically to cat's eye chrysoberyl, the gem species with the finest and most valued chatoyancy. Other species showing chatoyancy are qualified: "cat's eye tourmaline," "cat's eye quartz."

Microcrystalline quartz (SiO2) in which individual crystal grains are too small to see with the naked eye. The umbrella term for gem varieties including carnelian (orange-red), chrysoprase (green), agate (banded), onyx (black-white banded), and bloodstone (dark green with red spots).

The optical phenomenon producing a single bright band of light ("the eye") that moves across a cabochon surface as it is rotated. Produced by reflection from parallel needle-like inclusions, hollow tubes, or fibrous structures within the gem. The finest chatoyancy shows a sharp, well-defined eye. Gems showing chatoyancy include chrysoberyl (finest), quartz, tourmaline, apatite, and aquamarine.

A trace element or structural defect that selectively absorbs specific wavelengths of light, producing colour. In rubies and emeralds, chromium (Cr3+) is the chromophore. In blue sapphires, the Fe2+-Ti4+ charge transfer pair is the chromophore. In peridot, iron (Fe2+) is the chromophore. Understanding chromophores is the foundation of gem colour science.

Beryllium aluminium oxide (BeAl2O4). Mohs hardness 8.5 (one of the hardest gem minerals). Gem varieties include alexandrite (colour-change variety coloured by chromium), cat's eye chrysoberyl (chatoyant variety), and yellow chrysoberyl (golden yellow variety). Not to be confused with chrysoprase (which is quartz) or beryl (which is a different mineral).

The World Jewellery Confederation (French: Confédération Internationale de la Bijouterie, Joaillerie, Orfèvrerie des Diamants, Perles et Pierres). The international industry body that sets standards for gem nomenclature, quality grading, and treatment disclosure. The CIBJO Blue Books (separate volumes for diamonds, coloured stones, pearls, coral, and precious metals) are the primary international industry standards documents.

The Convention on International Trade in Endangered Species of Wild Fauna and Flora. A multilateral treaty controlling international trade in protected species. Appendix I prohibits commercial trade; Appendix II requires permits and documentation for sustainable trade. Relevant gem materials on CITES include all Corallium coral species (Appendix II) and some species of elephant ivory and tortoiseshell used historically as gem materials.

The tendency of a crystalline mineral to break along specific planar directions corresponding to planes of weak bonding in the crystal structure. Described by direction (basal, prismatic, octahedral) and quality (perfect, good, indistinct). Perfect cleavage (topaz, diamond, kunzite) means the mineral will split cleanly along this plane with a sharp blow; this is a significant care concern for jewellery. Gems with no significant cleavage (corundum, garnet, quartz) are more durable under impact.

The property of certain gems to appear significantly different in colour when viewed under different light sources (typically daylight/fluorescent vs incandescent light). The classic example is alexandrite (green in daylight, red in incandescent). Colour change is produced when the absorption spectrum of the gem has a transmission window that falls in different colour regions depending on the spectral distribution of the light source. Assessed by strength (strong, moderate, weak) and description of the two colours.

The organic protein matrix that binds aragonite platelets in pearl nacre and in mollusc shells. Comprises approximately 2-5% of nacre by weight. Degrades over centuries in buried or poorly stored pearls, causing them to lose their lustre and eventually disintegrate. One reason very old pearls from archaeological sites rarely retain their original appearance.

Aluminium oxide (Al2O3) crystallising in the trigonal system. The mineral species encompassing both ruby (red corundum, coloured by chromium) and sapphire (all other colours of gem corundum). Mohs hardness 9, second only to diamond. No significant cleavage. Specific gravity approximately 4.0. The combination of hardness, no cleavage, and wide colour range makes corundum the most commercially significant coloured gem mineral.

Describes a crystalline material whose individual crystals are so small (less than 1 micrometre) that they cannot be resolved even under optical microscopy. Chalcedony is cryptocrystalline quartz. The term is sometimes used interchangeably with microcrystalline, though microcrystalline technically refers to somewhat larger crystals visible under microscopy.

The green gem variety of andradite garnet, coloured by chromium. Has the highest dispersion of any gem mineral commonly used in jewellery (0.057, exceeding diamond's 0.044). Finest material comes from the Ural Mountains of Russia; characteristic diagnostic inclusions are horsetail actinolite fibre sprays around a chromite crystal. Mohs hardness approximately 6.5-7.

The property of a doubly refractive (uniaxial) gem to show two different colours when viewed from two different crystallographic directions. A form of pleochroism. The dichroscope (a simple optical instrument using calcite prisms) demonstrates dichroism by splitting the light from a gem into its two component colours simultaneously.

The optical phenomenon in which light is separated into its spectral components when it interacts with a regular periodic structure with spacing comparable to the wavelengths of light. Responsible for the play-of-colour in opal (silica sphere diffraction) and the colour of labradorite (thin-film diffraction from structural layers). Distinguished from dispersion (which requires refraction through a prism-like structure).

The property of a gem material to separate white light into its spectral colours as it passes through the stone, producing rainbow-coloured "fire." Measured as the difference in refractive index between the B (red, 686.7nm) and G (violet, 430.8nm) Fraunhofer spectral lines. Diamond: 0.044. Demantoid: 0.057. Zircon: 0.039. Sapphire: 0.018. Higher dispersion produces more fire.

An assembled gem consisting of two layers of different materials cemented together. A natural opal doublet has a thin natural opal layer cemented to a dark backing to enhance play-of-colour. A garnet-topped doublet has a genuine garnet crown over a glass pavilion. Commercially legitimate when disclosed; fraudulent when sold as a natural solid stone. Detection: view from the side; a join line is visible at the junction.

A simple optical filter that transmits only red and yellow-green light. Chromium-coloured emeralds appear red through the Chelsea filter because chromium transmits red light strongly. Not a definitive identification tool (some synthetic emeralds and some chromium tourmalines also appear red) but a useful screening indicator.

A gem is described as eye-clean when its inclusions are not visible to the unaided eye at normal viewing distance (approximately 25-30 cm). Not an official GIA grade, but a commonly used trade description. Eye-clean is not the same as flawless, even many SI1 and SI2 diamonds and fine ruby and sapphire are described as eye-clean while showing inclusions under magnification.

A flat polished surface cut on a gem. Facets are cut at specific angles to maximise light return (brilliance) and dispersion (fire). The main facets of a standard cut include the table (flat top facet), crown facets (above the girdle), girdle (widest perimeter), and pavilion facets (below the girdle). The exact number, shape, and angle of facets determines the cut style.

In diamond grading, any colour other than the D-Z (colourless to light yellow) scale. Includes intense yellow, orange, pink, red, blue, green, and other colours. Fancy colour diamonds are graded on a separate scale (Faint, Very Light, Light, Fancy Light, Fancy, Fancy Intense, Fancy Vivid). In coloured stone grading, "fancy colour" is not a formal term but sometimes used to distinguish vivid-coloured material from commercial grades.

The Chinese term specifically for jadeite jade (as distinct from nephrite jade). Literally meaning "translucent, colourful jade," feicui distinguished the superior jadeite arriving from Myanmar from the established nephrite tradition when Myanmar jadeite first reached China in significant quantities in the 18th century.

The emission of visible light by a gem when exposed to ultraviolet (UV) radiation. The emitted light disappears when the UV source is removed (distinguishing fluorescence from phosphorescence, which persists). In diamonds, fluorescence is commonly blue under long-wave UV. Some rubies fluoresce red (chromium fluorescence enhancing their apparent colour). Fluorescence can be a species indicator, an origin indicator, or a treatment indicator.

Fourier Transform Infrared Spectroscopy. An analytical technique that identifies molecular bonds by measuring the absorption of infrared light at different wavelengths. Used in gemology to detect polymer impregnation in jadeite (B jade), synthetic resins in emerald filling, and other organic treatments. A standard tool at major gem laboratories.

The world's largest and most widely recognised gemological laboratory and education institution, founded in 1931. Developer of the modern 4Cs diamond grading system and the GIA coloured stone colour and clarity grading methodology. Issues Colored Stone Identification Reports, Colored Stone Identification and Origin Reports, Diamond Grading Reports, and Pearl Reports. Certificates verified at gia.edu/report-check.

The Gem and Jewellery Export Promotion Council, the Indian government-supported industry body for the gem and jewellery trade, established in 1966. Primary source for Indian gem trade statistics, organiser of the India International Jewellery Show (IIJS), and the membership body for Indian gem exporters. GJEPC membership is a verifiable reliability signal for Indian dealers.

Historical diamond-mining region in the Deccan Plateau of India (modern Andhra Pradesh and Telangana). Source of all famous historical diamonds including the Koh-i-Noor, the Hope Diamond's predecessors, and the Orlov Diamond. The mines are essentially exhausted. "Golconda type" is used to describe large Type IIa (chemically pure) colourless diamonds with the physical profile consistent with Indian alluvial origin, commanding a premium at auction.

A calcium-aluminium garnet species (Ca3Al2(SiO4)3). Gem varieties include tsavorite (vivid green, coloured by vanadium and/or chromium), hessonite (honey orange-brown), and colourless to pale yellow grossular. The species name derives from the Latin for gooseberry (Ribes grossularium), referring to the green colour of some specimens.

One of the world's oldest and most prestigious gem laboratories, founded 1923 in Lucerne, Switzerland. Particularly associated with the finest Kashmir sapphire, Burmese ruby, and Russian alexandrite certifications. The Gübelin Photoatlas of Inclusions in Gemstones (three volumes) is the definitive inclusion identification reference. Gübelin certificates carry the highest prestige in the European auction market.

The application of high temperatures (typically 1,600-1,900°C for corundum) to a gem to improve or change its colour and/or clarity. The most widely applied gem treatment. Changes trace element oxidation states and dissolves silk (rutile needle inclusions). Accepted practice for ruby and sapphire; unheated stones command substantial premiums. Detected by microscopic examination of silk dissolution features and by LA-ICP-MS trace element analysis.

The orange-brown to honey-yellow variety of grossular garnet. Known for a characteristic "treacly" internal appearance under magnification. In Jyotish, hessonite is the Gomed, the prescribed gem for Rahu (north lunar node). Primary sources: Sri Lanka, India (Tamil Nadu), Brazil.

A diagnostic inclusion in Russian Ural demantoid garnet consisting of radiating sprays of actinolite fibres from a central chromite crystal, resembling a horse's tail under magnification. Unlike most inclusions, the horsetail in demantoid is considered a positive value indicator: its presence confirms Russian Ural origin and is specifically sought by collectors as proof of provenance.

In the GIA coloured stone colour system, the basic colour family of a gem (red, orange, yellow, green, blue, violet, purple). The first of the three dimensions of colour used in GIA grading, alongside tone (lightness/darkness) and saturation (colour purity/intensity). Hue modifiers describe secondary hues: "orangy red" has orange as a secondary hue to the primary red.

A porous variety of opal that absorbs water, temporarily reducing or eliminating its play-of-colour when wet and recovering the play-of-colour as it dries. Characteristic of most Ethiopian opal. Not permanently damaging but surprises uninformed buyers. The play-of-colour returns when the opal dries completely.

The trade designation for topaz in the orange to sherry-brown to pinkish-orange colour range, produced naturally by colour centres involving iron. The primary source is Ouro Preto, Minas Gerais, Brazil. Not an official grading term; the name "imperial" has no standardised definition and is applied inconsistently by dealers. The key quality factors are colour saturation, tone, and the absence of brownish or dull modifiers.

Any internal feature within a gem that differs from the host material. Inclusions may be solid (mineral crystals), liquid (fluid pockets), gaseous (bubbles), or two-phase or three-phase combinations. Some inclusions are diagnostic of species (rutile silk in corundum, actinolite horsetails in demantoid, lily pads in peridot) or of origin (specific mineral assemblages associated with specific deposits). Inclusions are assessed in clarity grading.

A colour-producing mechanism in which an electron transfers between two adjacent ions (e.g., Fe2+ and Ti4+ in blue sapphire) when light of a specific wavelength is absorbed. The resulting colour depends on the wavelength absorbed. The Fe2+-Ti4+ charge transfer pair in corundum produces blue sapphire colour. Distinguished from crystal field theory (single-ion absorption), which explains chromium colours in ruby and emerald.

Exposure of a gem to nuclear radiation (gamma rays, neutrons, or electrons) to create or modify colour centres. Used commercially to produce blue topaz from colourless topaz, to deepen some tourmaline colours, and to colour diamonds. Blue topaz irradiation is universally accepted in the trade. Neutron-activated London Blue topaz requires safety testing before sale. Detected by GIA and laboratories using UV-Vis spectroscopy and other methods.

From the French for "garden," the characteristic inclusions of emerald, a complex mixture of two-phase and three-phase fluid inclusions, fractures, and other features that collectively give most emeralds their distinctive internal appearance. An accepted characteristic of natural emerald; the trade describes emerald clarity in terms of jardin extent rather than by standard clarity grades. A stone without any jardin is either synthetic or exceptionally rare natural material.

Vedic astrology; the traditional Indian astrological system. Jyotish gem prescriptions assign specific gem species to each of the nine Navagraha (celestial bodies) and prescribe wearing specific gems to strengthen or propitiate planetary influences identified in an individual's birth chart. The Navratna (nine gems) system is the foundation of Jyotish gem use. Gem quality requirements in the classical texts are demanding: natural origin, good colour, adequate clarity, free from significant fractures.

Blue sapphire from the Padar area of Jammu and Kashmir, India, mined most significantly between approximately 1881 and 1925. The benchmark for the finest blue sapphire in the world; the specific silky-blue appearance with velvety quality is produced by microscopic rutile inclusions that scatter light, reducing the stone's transparency while giving the blue a distinctive depth. The deposit has not produced significant new material since approximately 1925, making Kashmir sapphires among the most supply-constrained gems in existence.

A pearl formed by accident during the culturing process, when the mollusc rejects the nucleus but continues producing nacre. Keshi pearls are entirely nacre (no nucleus) and occur in irregular baroque shapes. Technically not cultured (no intended nucleation) but also not natural (produced in a farm setting). Some laboratories classify keshi differently; confirm classification before any significant purchase.

Laser Ablation Inductively Coupled Plasma Mass Spectrometry. The primary analytical instrument for trace element analysis in gemology. Used to detect beryllium diffusion, lead glass filling, synthetic vs natural determination in some species, and geographic origin (by comparison of trace element profiles against reference databases). A standard tool at GIA and major laboratories.

A treatment in which highly fractured, low-quality corundum is heated with lead-rich glass that flows into the fractures, producing apparent clarity. The glass content can be 20-60% of total volume. Physically fragile (heat-sensitive, acid-sensitive), commercially worthless compared to natural ruby, and must be disclosed. Detected by the flash effect under fibre-optic light and by LA-ICP-MS lead detection.

A characteristic partially healed fracture in peridot with a curved outline resembling a lily pad on water. Produced when a fracture in the crystal partially heals during growth, leaving a distinctive curved surface. Diagnostic for peridot; not present in other common green gems.

A small handheld magnifying lens used in gem examination. The standard gemological loupe is 10x magnification (ten-power), triplet corrected (three-element lens correcting for chromatic and spherical aberration). GIA clarity grading is defined at 10x magnification. "Eye-clean" means no inclusions visible without a loupe.

The quality and character of light reflected from a gem's surface. Described as adamantine (diamond-like, highest), vitreous (glassy, most common), resinous (resin-like), waxy, silky, pearly, or metallic. Pearl's specific lustre is characterised separately as the product of thin-film interference in nacre layers. Lustre is the primary quality factor for pearl evaluation.

Describes a mineral whose crystal structure has been progressively damaged by radioactive decay of uranium and thorium within the crystal, converting the originally ordered crystal lattice toward an amorphous state. Low zircon is the most commercially relevant metamict gem material. Heat treatment can partially restore crystallinity in metamict zircon.

A naturally occurring, inorganic substance that does not meet the full definition of a mineral due to lack of crystalline structure. Opal is the most commercially significant mineraloid gem material. Amber (organic origin) and pearl (biogenic origin) are not mineraloids but are similarly excluded from strict mineral classification.

The ruby and sapphire mining district in Mandalay Region, upper Myanmar (Burma). The world's most celebrated coloured gem mining district, producing the finest ruby in the world over approximately 1,600 years of documented history. Also produces blue sapphire, spinel, moonstone, alexandrite, and other gem species. The "Mogok stone tract" is the legal term for the mining area.

A relative hardness scale from 1 (talc) to 10 (diamond), developed by Friedrich Mohs in 1812. Each mineral can scratch all minerals below it on the scale and is scratched by all minerals above it. Gemologically, hardness above 7 is generally considered durable for jewellery; below 7, the gem is susceptible to scratching from common abrasives (dust, which contains quartz at Mohs 7). Not a linear scale: the hardness difference between 9 (corundum) and 10 (diamond) is much larger than between 1 and 9.

The iridescent material secreted by molluscs to form pearls and line shells. Composed of flat hexagonal aragonite platelets (approximately 0.4 micrometres thick) bound by conchiolin protein. The stacking of thousands of thin platelets produces thin-film interference, creating pearl's characteristic lustre. Nacre thickness is a primary quality factor: Akoya pearls have approximately 0.35-0.7mm; South Sea pearls have 2-6mm.

The nine celestial bodies recognised in Jyotish (Vedic astrology): Surya (Sun), Chandra (Moon), Mangal (Mars), Budha (Mercury), Guru/Brihaspati (Jupiter), Shukra (Venus), Shani (Saturn), Rahu (north lunar node), and Ketu (south lunar node). Each Navagraha governs specific life domains and is associated with a specific gem in the Navratna system.

Sanskrit for "nine gems." The system in Jyotish that assigns one gem species to each of the nine Navagraha planets. The nine gems: ruby (Sun), pearl (Moon), coral (Mars), emerald (Mercury), yellow sapphire (Jupiter), diamond (Venus), blue sapphire (Saturn), hessonite garnet (Rahu), cat's-eye chrysoberyl (Ketu). Documented in classical texts including the Brhat Samhita and Garuda Purana from approximately 550-1000 CE.

A silicate mineral in which individual SiO4 tetrahedra are isolated (not sharing oxygen with adjacent tetrahedra). Garnets, topaz, olivine (peridot), and zircon are nesosilicates. The term means "island silicate" (Greek nesos, island). Contrasted with framework silicates (quartz, feldspar), chain silicates (pyroxenes, amphiboles), and ring silicates (beryl, tourmaline).

A magnesium-iron silicate mineral group ((Mg,Fe)2SiO4) of which the gem variety is peridot. The most abundant mineral in the earth's upper mantle. The forsterite end-member (pure Mg2SiO4) is colourless; the gem peridot is the intermediate fayalite-forsterite solid solution where iron content produces the yellow-green colour.

The iridescent shimmer visible on fine pearl surfaces when the pearl is moved, a shifting play of spectral colours (pinks, blues, greens, golds) produced when light at different wavelengths is selectively enhanced by thin-film interference in the nacre layers. Orient is most pronounced in fine South Sea and Tahitian pearls with particularly regular nacre. Distinguished from lustre (which is the sharpness of reflection) and from adularescence (which is in feldspar gems).

The laboratory process of determining the geographic source of a gem using a combination of trace element analysis (LA-ICP-MS), stable isotope analysis, microscopic examination of inclusions, and comparison against reference databases of known-origin material. Only GIA, AGL, Gübelin, SSEF, and Lotus Gemology origin determinations are commercially recognised at major international auction houses. Origin premiums require current major laboratory origin certificates.

The most valuable sapphire variety; a pinkish-orange to orangish-pink sapphire from Sri Lanka (Ceylon). The name comes from the Sinhalese for "lotus blossom." There is no universally agreed definition of what constitutes padparadscha vs pink sapphire or orange sapphire; GIA describes the colour range as occupying an intermediate zone between pink and orange. Padparadscha commands significant premiums over equivalent-quality pink or orange sapphire.

Copper-bearing tourmaline first discovered in Paraiba state, Brazil in 1987, producing neon blue-green to vivid green colours unlike any other gem. The colour is produced by copper (Cu2+) and manganese (Mn2+). Secondary deposits in Mozambique and Nigeria also produce copper-bearing tourmaline. GIA and other laboratories require copper confirmation for Paraiba designation; origin (Brazilian vs African) affects price significantly.

An exceptionally coarse-grained igneous rock, typically granitic in composition, that forms in the late stage of magma crystallisation when water and other fluids concentrate rare elements. The geological source of most gem beryl (emerald, aquamarine, morganite), tourmaline, topaz, and spodumene (kunzite). Pegmatite gems form in cavities and pockets where elements like beryllium, lithium, and caesium concentrate to form unusual minerals.

The trade term for the finest quality Burmese ruby colour: a vivid, saturated red with a slight blue undertone (from chromium fluorescence) and no brownish or orange modifier. The name comes from the traditional comparison to the deep red colour of the first drops of blood from a sacrificed pigeon. Not a formally standardised colour grade; used variably by dealers and occasionally loosely. GIA describes the colour in objective hue-tone-saturation language without using this term on certificates.

The spectral colour display in precious opal produced by diffraction of white light by regular arrays of silica spheres. Distinguished from iridescence (which is produced by thin-film interference) by being a diffraction phenomenon. The colours produced depend on the sphere spacing: approximately 150nm spacing produces blue/violet; approximately 250nm produces red/orange. Play-of-colour that includes red is rarest and most valuable.

The property of doubly or triply refractive coloured gems to show different colours when viewed from different crystallographic directions. Dichroism (two colours, in uniaxial crystals) and trichroism (three colours, in biaxial crystals) are the specific forms. Tanzanite is strongly trichroic (blue-violet, purple, and green from three directions). Andalusite is strongly trichroic (yellow-green, red, and green). Used as a species identification aid.

Traditional Indian unit of gem weight derived from the weight of the seeds of the Abrus precatorius (rosary pea or ratti plant). One ratti is approximately 0.12 grams or approximately 0.6 metric carats. Used in the Jyotish tradition to specify gem weight requirements; classical texts typically recommend 3-5 rattis (approximately 1.8-3 carats) for major planetary gems.

The primary gem identification instrument, measuring a gem's refractive index (RI) by determining the critical angle at which light undergoes total internal reflection. A flat polished surface is required. The RI reading identifies the gem species (sapphire 1.76-1.77, emerald 1.57-1.58, diamond 2.42, quartz 1.54-1.55) and immediately distinguishes simulants from genuine gems. A basic gem purchase protection tool available to any dealer.

The ratio of the speed of light in a vacuum to its speed in the gem material. Numerically equal to the sine of the angle of incidence divided by the sine of the angle of refraction. Higher RI means light slows more in the material, bends more at the surface, and produces more brilliance. Diamond's RI (2.42) is one of the highest of any transparent material. RI is the primary gem identification measurement.

A pyrope-almandine garnet solid solution (approximately 2:1 pyrope to almandine) producing raspberry red to purplish red colour. Named from the Greek for rose. Found in Tanzania (Umba Valley), Zimbabwe, India, and other locations. Commercially attractive, accessible in large sizes, and well within the affordable gem segment.

Red corundum (Al2O3) coloured primarily by chromium (Cr3+) substituting for aluminium. The most valuable coloured gemstone per carat at the finest quality tier. The distinction between ruby and pink sapphire (where the same species begins to be called sapphire rather than ruby) is a subject of ongoing debate in the trade; GIA uses a saturation threshold and Beyer scale colour standards. Sources: Mogok Burma (finest), Mozambique (commercial), Madagascar, Tanzania, Vietnam, Thailand.

Needle-like inclusions of rutile (TiO2) in corundum (ruby and sapphire), arranged in three directions at 120 degrees to each other following the crystal structure. In sufficient density, rutile silk produces a silky or hazy appearance. When controlled, rutile silk produces asterism (star effect). Partially or fully dissolved silk is a key indicator of heat treatment. Also called "silk" in the gem trade.

A trade designation for the deepest, most saturated pure blue aquamarine colour, named after the Santa Maria de Itabira mine in Minas Gerais, Brazil that historically produced the benchmark material. The mine is no longer in active production; "Santa Maria colour" now refers to any aquamarine approaching this quality standard regardless of origin. Not a laboratory grade; a commercial description.

In the GIA colour system, the intensity or purity of colour, ranging from grey/brown (no saturation) to vivid (maximum saturation for the given hue and tone). "Vivid" is the highest saturation grade and requires strong colour intensity without grey or brown modifiers. A vivid-saturation stone is the finest possible for its colour; grayish or brownish modifiers reduce commercial value significantly.

A play of colours in labradorite and other feldspars produced by thin-film interference between alternating layers of different feldspar compositions. Distinguished from adularescence (which is in moonstone and is more of a glowing light than a colour play) and from iridescence generally. The term "labradorescence" is also used for the schiller effect in labradorite.

See: rutile silk. In the gem trade, "silk" specifically refers to the rutile needle inclusions in ruby and sapphire. The term comes from the silky haze these inclusions produce in sufficient density. Intact silk indicates an unheated stone; dissolved or partially dissolved silk is a heat treatment indicator.

The ratio of a gem's weight to the weight of an equal volume of water. A key identification property measured by hydrostatic weighing (weighing in air and in water) or by heavy liquid comparison. Diamond SG 3.52; corundum SG 4.0; quartz SG 2.65; amber SG approximately 1.08 (floats in salt water). SG distinguishes between visually similar species and is particularly useful for assembled stone detection.

Manganese aluminium garnet (Mn3Al2(SiO4)3). The gem variety producing orange to orange-red colour; the vivid pure orange variety from Namibia is known as "mandarin garnet." One of the most distinctive orange gems available with no treatment required. Sources: Namibia (Loliondo), Nigeria, Madagascar.

A major Swiss gem laboratory based in Basel, founded 1974. Particularly known for pearl X-ray identification (the standard method for natural vs cultured pearl determination), alexandrite expertise, and fine coloured stone origin determination. SSEF certificates are accepted at all major international auction houses. Verification at ssef.ch/verification.

Sapphire or ruby displaying asterism (a star effect) produced by three sets of rutile needle inclusions at 120 degrees, creating a six-rayed star when cut as a cabochon and illuminated from above with a single point light source. Black star sapphire (dark body colour with silver star) from Thailand and Australia is also commercially significant. The star must be sharp, centred, and well-defined for fine quality.

The specific variety name for Baltic amber, technically the only variety of amber that is properly called succinite. Characterised by a succinic acid content of 3-8% by weight, which distinguishes it chemically from other amber varieties. Approximately 34-40 million years old. The primary commercial amber variety globally.

A disc-shaped stress fracture produced in amber during the clarification treatment, when trapped air bubbles are eliminated by heating under pressure. The differential thermal expansion at the bubble site produces these characteristic circular fractures. Sun spangles are diagnostic of clarified amber and are visible to the naked eye in transmitted light as disc-shaped features with concentric rings.

A gem material grown in a laboratory that has the same chemical composition, crystal structure, and physical properties as the natural gem. Synthetic ruby, sapphire, emerald, alexandrite, and diamond exist commercially. Synthetic gems are chemically identical to natural gems but are not natural. They require laboratory testing to distinguish from natural material and must be disclosed. Price is typically 1/10 to 1/100 of equivalent natural material.

The blue-violet gem variety of the mineral zoisite (Ca2Al3(SiO4)3(OH)), coloured by vanadium. Found exclusively in a small mining area near Arusha, Tanzania (Merelani Hills). Shows strong pleochroism (blue-violet, purple, and green from different directions). Virtually all commercial tanzanite has been heat treated to produce the blue-violet colour from the brownish rough. Single-source supply creates inherent supply constraint.

An optical phenomenon in which light reflecting from the top and bottom surfaces of a thin transparent layer travels slightly different path lengths, causing specific wavelengths to be reinforced (constructive interference) or cancelled (destructive interference). Produces the colours of soap bubbles, oil films on water, and pearl lustre (from nacre platelet layers). The wavelength reinforced depends on the film thickness and viewing angle.

In the GIA colour system, the lightness to darkness of a gem colour on a scale from 0 (colourless) to 10 (black). Most fine coloured gems have optimal tone in the range 5-7 (medium to medium-dark). Tones below 3 are too pale (washed out); tones above 8 are too dark (opaque or near-opaque). Tone interacts with saturation: a saturated colour at appropriate tone is fine quality; the same colour at too-dark tone appears nearly black.

A complex boron silicate mineral group producing the widest colour range of any single gem mineral, all colours including the only gem-quality blue-green produced by copper (Paraiba tourmaline). Hardness 7-7.5; no significant cleavage. Major commercial varieties: Paraiba (copper, neon blue-green), rubellite (red), indicolite (blue), verdelite (green), watermelon (pink and green bicolour), chrome tourmaline (chromium green).

The property of biaxial doubly refractive gems to show three different colours when viewed from three different crystallographic directions. Tanzanite is one of the most strongly trichroic gem minerals (blue-violet, purple, and green). Andalusite also shows strong trichroism. Trichroism requires a dichroscope for demonstration and is a gem identification and cutting orientation tool.

The vivid green variety of grossular garnet, coloured by vanadium and/or chromium. Discovered in the Tsavo National Park area of Kenya in the late 1960s by Campbell Bridges and named by Tiffany and Company. Visually competitive with fine emerald at the finest quality, without emerald's treatment complexity. Nearly always under 1 carat in commercial production; fine material above 2 carats is genuinely rare.

GIA's classification of coloured gems by what is typical for the species regarding inclusions. Type I (typically inclusion-free): aquamarine, tanzanite, citrine. Type II (typically included): ruby, sapphire, alexandrite, spinel. Type III (almost always included): emerald, rubellite. The classification sets the baseline expectation for clarity in each species: eye-clean is normal for Type I, a premium for Type II, and exceptional for Type III.

Description of a gem that shows no evidence of heat treatment on laboratory examination. For ruby and sapphire, "no indications of heating" on a GIA, AGL, Gübelin, or SSEF certificate is the standard commercial confirmation of unheated status. Unheated corundum commands substantial price premiums because heat-treated material is dominant in commercial production. The unheated designation is an assessment, not an absolute guarantee: very low-temperature or evidence-free treatment may not be detectable.

The first commercial synthetic corundum production method, developed by Auguste Verneuil in 1902. Aluminium oxide powder is melted by a high-temperature flame and deposited onto a growing crystal. Produces synthetic ruby and sapphire of large size quickly and inexpensively. Diagnostic features include curved colour bands and spherical gas bubbles visible under microscopy. The process that first brought synthetic gem corundum to the commercial market.

The highest saturation grade in GIA's coloured stone colour system. Indicates a colour that is strongly saturated with the minimum of grey or brown modification. "Vivid" is the most commercially significant colour grade word in coloured stone grading: GIA "vivid red" ruby or GIA "vivid blue" sapphire identifies the finest colour tier. Not all rubies or sapphires receive a vivid grade; moderate-saturation material is graded "strong" or lower.

A zone of transparency visible through the table facet of a cut coloured stone, where the colour fails to develop and the viewer can see through the stone as through glass. Produced by incorrect cut proportions (usually too shallow a pavilion) that allow light to pass through the stone rather than reflecting back to the eye. A window significantly reduces the apparent colour quality and commercial value of a coloured stone.

Uneven distribution of colour within a gem, produced by changes in trace element concentration during crystal growth. Common in sapphire (angular blue zones), amethyst (usually concentrated at crystal tips), and tourmaline (may show concentric zoning or axial colour variation). Pronounced colour zoning reduces commercial value; even colour distribution commands premiums. Skilled cutting can use zoning to advantage by positioning the most saturated zone under the table facet.

Natural zirconium silicate (ZrSiO4), a tetragonal nesosilicate. Not to be confused with cubic zirconia (ZrO2, a synthetic material). Zircon has exceptional optical properties (RI 1.925-1.984, dispersion 0.039) and contains uranium making it the world's most important geochronological mineral. The oldest known terrestrial material (Jack Hills, Australia, 4.404 billion years) is detrital zircon. Blue zircon from Cambodia, produced by heat treatment, is the most commercially significant gem variety.