Glossary

Also called: Four Cs

The four criteria used to judge the quality of a diamond: colour, clarity, cut and carat weight. Popularised by GIA (the Gemological Institute of America) in the mid-20th century, the system is now loosely applied to coloured stones as well.

Related terms: Carat, Cut, Inclusion, GemstoneExample stones: Diamond

Also called: Adamantine luster

A brilliant, sparkling lustre, much brighter than vitreous, named after diamond and seen in minerals with a high refractive index. Diamond, zircon, cinnabar and some sphalerite have an adamantine lustre.

Related terms: Lustre, Refractive indexExample stones: Diamond, Zircon, Cinnabar, Sphalerite

A soft bluish or white glow that seems to float inside moonstone and moves as the stone is turned. It is caused by light scattering from thin, alternating layers of orthoclase and albite feldspar, and it is named after adularia, a form of feldspar.

Related terms: Labradorescence, Opalescence, CabochonExample stones: Moonstone, Orthoclase

Describes a solid whose atoms are arranged at random rather than in an ordered crystal structure. Amorphous materials never grow crystal faces and usually break with a conchoidal fracture; volcanic glass (obsidian), opal and amber are examples.

Related terms: Mineraloid, Crystal structure, Volcanic glass, Conchoidal fractureExample stones: Obsidian, Opal, Amber

The trade name for six silicate minerals that separate into fine, heat- and chemical-resistant fibres: chrysotile (white asbestos) from the serpentine group, and crocidolite, amosite, anthophyllite, tremolite and actinolite from the amphibole group. Breathing in the fibres causes asbestosis, lung cancer and mesothelioma; the use of asbestos has been banned in Türkiye since the end of 2010.

Related terms: Silicosis, SilicateExample stones: Chrysotile (white asbestos), Serpentinite

Also called: Star effect

A star-shaped pattern of bright lines that appears on the surface of some stones cut as cabochons. It is caused by fine needle-like inclusions (most often rutile) lined up in several directions inside the stone; star sapphires and star rubies usually show a six-rayed star.

Related terms: Chatoyancy, Inclusion, CabochonExample stones: Sapphire, Ruby

Also called: Double refraction

The splitting of light into two rays that travel at different speeds in some crystals; the difference between the two refractive indices is its measure. Clear calcite (Iceland spar) shows it so strongly that writing seen through it appears double. Cubic minerals and amorphous materials are not birefringent.

Related terms: Refractive index, Pleochroism, Cubic systemExample stones: Calcite

Also called: Borate minerals

The mineral class containing borate groups, made of boron and oxygen. Borax (tincal), ulexite and colemanite are the most important borates; the world's largest known boron reserves are in Türkiye, and these minerals are raw materials for glass, glass fibre, ceramics and cleaning products.

Related terms: Evaporite, Ore depositExample stones: Borax (tincal), Ulexite, Colemanite

The tendency of a mineral to break into fragments or powder, rather than bend, when it is struck, crushed or cut. Most minerals, such as quartz, pyrite and calcite, are brittle; by contrast, native metals such as gold, silver and copper are malleable, mica flakes are elastic and talc can be cut with a knife (it is sectile).

Related terms: Malleability, Toughness, Fracture, HardnessExample stones: Quartz, Pyrite, Muscovite, Talc

A gem cut with a smooth, rounded dome and no facets. It is used for opaque and translucent stones and for stones that show optical effects such as stars and cat's eyes; turquoise, moonstone, tiger's eye, opal and star sapphire are usually cut as cabochons.

Related terms: Cut, Facet, Asterism, Chatoyancy, AdularescenceExample stones: Turquoise, Moonstone, Tiger's eye, Opal

Also called: ct

The unit of weight for gemstones: one carat is 0.2 grams (200 milligrams) and is divided into 100 points; the name is thought to come from the carob seed. The purity of gold is also measured in carats (spelled karats in American English), but on a different scale on which 24 is pure gold.

Related terms: 4Cs, GemstoneExample stones: Diamond, Gold

Also called: Carbonate minerals

The mineral class containing the carbonate (CO₃) group. Calcite, dolomite, malachite and azurite belong here; calcite fizzes in dilute acid (even vinegar will do), a simple test used to recognise carbonates. Limestone, marble and travertine are made mainly of carbonate minerals.

Related terms: Sedimentary rock, Commercial marbleExample stones: Calcite, Dolomite, Malachite, Azurite, Limestone

Also called: Cat's-eye effect

A single bright band of light, like the slit pupil of a cat's eye, that glides across a cabochon-cut stone as it is turned. It is caused by fine fibres or needle-like inclusions lying parallel inside the stone; tiger's eye is the best-known example.

Related terms: Asterism, Inclusion, CabochonExample stones: Tiger's eye

A stony meteorite containing tiny rounded grains called chondrules. These rocks, left over from the formation of the Solar System about 4.56 billion years ago and never melted as a whole, make up the great majority of meteorites that fall to Earth; carbonaceous chondrites also contain water and organic compounds.

Related terms: Chondrule, MeteoriteExample stones: Chondrite

Rounded grains, mostly about a millimetre across, found in chondrites. They are droplets of olivine, pyroxene and glass that formed when clumps of dust in the early Solar System were briefly melted and then cooled quickly; what heated them is still debated.

Related terms: Chondrite, MeteoriteExample stones: Chondrite, Olivine

The tendency of a mineral to split along weak planes in its crystal structure, leaving flat, shiny surfaces. Mica splits into thin sheets, calcite into rhombs and halite into cubes.

Related terms: Fracture, Crystal structure, ToughnessExample stones: Muscovite, Calcite, Halite (rock salt)

Also called: Granite (trade name)

In the stone trade, the name given to almost any crystalline stone that is harder than marble and takes a polish. True granite is a coarse-grained plutonic rock rich in quartz and feldspar, but most stone sold as “black granite” is gabbro or a similar dark rock, and metamorphic rocks such as gneiss are also sold as granite.

Related terms: Commercial marble, Plutonic rock, QuarryExample stones: Granite, Gabbro, Gneiss

Also called: Marble (trade name)

In the stone trade, the name given to almost any relatively soft stone that takes a polish. In geology, marble is only the rock formed when limestone or dolomite recrystallises during metamorphism, but on the market limestone, travertine, serpentinite and banded calcite (“onyx marble”) are also sold as marble.

Related terms: Quarry, Metamorphism, Commercial granite, Onyx marble, CarbonateExample stones: Marble, Limestone, Travertine, Serpentinite

Also called: Shell-like fracture

Breakage that leaves smooth, curved surfaces with concentric ripples, like the inside of a mussel shell. It is seen in amorphous materials such as glass and obsidian and in minerals without cleavage such as quartz and flint; Stone Age people made use of it to knap sharp tools.

Related terms: Fracture, Amorphous, Volcanic glassExample stones: Obsidian, Chert and flint, Quartz

A solid whose atoms are arranged in an orderly pattern that repeats in three dimensions. This inner order can show on the outside: crystals that grow freely, with enough space, take flat-faced geometric shapes such as the six-sided prisms of quartz or the cubes of pyrite.

Related terms: Crystal structure, Crystal system, Crystal habit, AmorphousExample stones: Quartz, Pyrite

Also called: Habit

The outward shape that a mineral's crystals or crystal aggregates usually take. Tourmaline forms long prisms, kyanite flat blades, pyrite cubes, malachite rounded, grape-like (botryoidal) masses and chrysotile fine fibres.

Related terms: Crystal, Crystal system, TwinningExample stones: Tourmaline, Kyanite, Pyrite, Malachite, Chrysotile (white asbestos)

Also called: Crystal lattice

The regular arrangement of atoms or ions in space within a crystal. Even the same element can build different structures: diamond and graphite are both carbon, but because their atoms are arranged differently, one is the hardest mineral and the other is soft enough to mark paper.

Related terms: Crystal, Crystal system, Polymorph, CleavageExample stones: Diamond, Graphite

One of the seven groups into which crystals are divided by their symmetry and by the lengths and angles of their crystal axes: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic. A mineral's crystal system determines the shapes its crystals can take.

Related terms: Cubic system, Tetragonal system, Hexagonal system, Trigonal system, Orthorhombic system, Monoclinic system, Triclinic system, Crystal structure

Also called: Isometric system

The crystal system with the highest symmetry: three crystal axes of equal length, all at right angles to one another. Cubes, octahedra and dodecahedra belong here; halite, pyrite, galena, fluorite, garnet and diamond crystallise in the cubic system.

Related terms: Crystal system, Crystal habitExample stones: Halite (rock salt), Pyrite, Galena, Fluorite, Garnet, Diamond

The shape and arrangement of surfaces given to a gemstone, and the quality of that work. Faceted cuts such as the round brilliant reflect light inside the stone to create brilliance and fire, while a cabochon is dome-shaped. A good cut makes the most of a stone's proportions, symmetry and polish.

Related terms: Facet, Cabochon, 4Cs, DispersionExample stones: Diamond

A mineral classification based on James Dwight Dana's System of Mineralogy, first published in 1837, and revised in Dana's New Mineralogy (1997). It divides minerals into 78 classes by chemistry and structure and labels them with numbers such as 75.1.3.1 (quartz); in this system quartz is a silicate.

Related terms: Nickel–Strunz classification, SilicateExample stones: Quartz

The mass of a unit volume of a substance, usually given in g/cm³ for minerals. It depends on how tightly the crystal structure is packed and how heavy its elements are: minerals containing heavy elements, such as baryte and galena, feel surprisingly heavy in the hand, while porous pumice can float on water.

Related terms: Specific gravityExample stones: Baryte, Galena, Pumice

All the physical, chemical and biological changes that sediments undergo after they are laid down, at low temperature and pressure. Through compaction, the growth of calcite, quartz or iron oxide cement between grains, and recrystallisation, loose sand becomes sandstone and mud becomes shale; at higher temperature and pressure the process gives way to metamorphism.

Related terms: Sedimentary rock, Metamorphism, Rock cycleExample stones: Sandstone, Shale

Also called: Fire

The splitting of white light into the colours of the rainbow as it passes through a stone; in gems it shows as flashes of coloured light, known as fire. Diamond's strong dispersion is a large part of its appeal, and zircon also shows distinct fire.

Related terms: Refractive index, Facet, CutExample stones: Diamond, Zircon

A composite stone made by gluing two pieces together. In an opal doublet a thin slice of precious opal is glued onto a dark backing, while doublets with a thin garnet layer fused onto coloured glass were once widely used to imitate many gems. The join can often be seen from the side.

Related terms: Triplet, ImitationExample stones: Opal, Garnet

Soaking porous or fractured stones in dye to give them colour or deepen it. Agate and chalcedony have been dyed for centuries (most black “onyx” is dyed agate or chalcedony), and some pale stones are dyed to imitate turquoise. The dye can fade over time or be wiped off with acetone.

Related terms: Gemstone treatment, Imitation, Onyx marbleExample stones: Agate, Chalcedony, Onyx, Turquoise

The removal and transport of rock fragments and soil, loosened by weathering, by rivers, wind, glaciers, waves and gravity. The material is laid down elsewhere and becomes the raw material of sedimentary rocks; the fairy chimneys of Cappadocia are landforms shaped by erosion.

Related terms: Weathering, Sedimentary rock, Rock cycle, Placer depositExample stones: Tuff

Minerals and rocks formed when dissolved salts settle out as sea, lake or lagoon water evaporates. Halite (rock salt), gypsum and anhydrite are the commonest evaporites, and salt still forms this way today in Lake Tuz in central Türkiye. Türkiye's deposits of borax, ulexite and colemanite also settled out in ancient lakes that evaporated in a dry climate.

Related terms: Sedimentary rock, Borate, Halide, SulfateExample stones: Halite (rock salt), Gypsum, Borax (tincal), Ulexite, Colemanite

Each of the flat, polished surfaces of a cut gemstone. A round brilliant usually has 57 or 58 facets, the largest one on top being called the table; the angles of the facets make light reflect inside the stone and return to the viewer.

Related terms: Cut, Cabochon, DispersionExample stones: Diamond

The emission of visible light by a substance that absorbs invisible radiation such as ultraviolet (UV) light; it stops when the light source is switched off. It is named after fluorite, and some fluorite, calcite and diamond glow in colour under a UV lamp. A glow that lasts after the source is switched off is called phosphorescence.

Example stones: Fluorite, Calcite, Diamond

A layered or sheet-like fabric in metamorphic rocks, formed when flat or elongated minerals such as mica line up parallel to one another under pressure. The way slate splits into thin sheets, the flaky sparkle of schist and the light and dark bands of gneiss show different degrees of foliation, while marble and quartzite are usually unfoliated.

Related terms: Metamorphic rock, Metamorphism, CleavageExample stones: Slate, Schist, Gneiss

The preserved remains (bones, shells, leaves) or traces (footprints, burrows) of organisms that lived in past geological ages. Most are found in sedimentary rocks; amber, which is fossilised tree resin, and Oltu stone, formed from coalified wood, are stones of fossil origin.

Related terms: Sedimentary rock, Organic gem, InclusionExample stones: Amber, Oltu stone, Limestone, Coal

Breakage along irregular surfaces, in a mineral that has no cleavage or breaks in a direction other than its cleavage. By shape it is described as conchoidal (shell-like), uneven, fibrous, splintery or hackly; metals such as native copper break with hackly, jagged surfaces.

Related terms: Cleavage, Conchoidal fracture, BrittlenessExample stones: Quartz, Copper

Also called: Clarity enhancement, Oiling

Filling a stone's surface-reaching cracks and cavities with glass, resin or oil to make them less visible. Most emeralds are filled with oil or resin, and lead-glass-filled low-grade rubies and fracture-filled diamonds are also on the market. Fillings can be damaged by heat, ultrasonic cleaning and chemicals.

Related terms: Gemstone treatment, InclusionExample stones: Emerald, Ruby, Diamond

A thin black coating, usually less than a millimetre thick and glassy or matt, that forms when the outer surface of a meteorite melts during its passage through the atmosphere and then solidifies. It is jet black on freshly fallen meteorites and turns brown as it rusts; on a stone with a lighter interior it is an important sign of a meteorite.

Related terms: Meteorite, MeteorExample stones: Chondrite, Iron meteorite

Also called: Gangue mineral

The minerals that accompany the ore minerals in a deposit and have no value to the mine. Quartz, calcite, baryte and fluorite are common gangue minerals in metal veins and are separated from the ore during processing. However, baryte or fluorite that is gangue in one deposit may be mined for its own sake elsewhere.

Related terms: Ore, Vein, Ore depositExample stones: Quartz, Calcite, Baryte, Fluorite

Also called: Gem

A mineral, rock or organic material that is cut and polished for jewellery or ornaments because of its beauty, durability and rarity. Besides minerals such as diamond, ruby, sapphire and emerald, rocks such as lapis lazuli and organic materials such as pearl and amber count as gemstones.

Related terms: Semi-precious stone, 4Cs, Organic gem, Synthetic gemstoneExample stones: Diamond, Ruby, Lapis lazuli, Pearl

Also called: Enhancement, Treated gemstone

Any process beyond cutting and polishing that is used to improve a gemstone's colour, clarity or durability, such as heat treatment, irradiation, dyeing, fracture filling or coating. Many treatments are common and accepted, but they must be disclosed to the buyer, and some are not permanent.

Related terms: Heat treatment, Irradiation, Dyeing, Fracture filling, Synthetic gemstone

A rounded, hollow cavity in rock whose inner walls are lined with crystals. Most form in gas bubbles in lava or in cavities in sedimentary rock, as water seeping in deposits minerals layer by layer; geodes of amethyst, quartz, calcite and celestine are well known. Ones that are completely filled are called nodules.

Related terms: Vein, HydrothermalExample stones: Amethyst, Agate, Calcite, Celestine

Also called: Origin, Provenance

The country or region where a gemstone was mined. Some origins, such as Kashmir sapphire, Burmese (Myanmar) ruby and Colombian emerald, noticeably affect a stone's value; gem laboratories assess origin from inclusions and trace elements, but the result is an expert opinion, not always a certainty.

Related terms: Inclusion, GemstoneExample stones: Sapphire, Ruby, Emerald

Also called: UNESCO Global Geopark

A defined area with geological heritage of international importance, managed so as to combine its protection with education and sustainable development. UNESCO Global Geoparks have been an official UNESCO programme since 2015; Türkiye's first is the Kula-Salihli geopark in Manisa (2013), known for its young volcanic cones and lava flows.

Related terms: Lava, Volcanic rockExample stones: Basalt

Also called: Halide minerals

The mineral class formed when halogen elements such as chlorine, fluorine, bromine or iodine combine with metals. Halite (rock salt) and fluorite are the best-known halides; most are soft, and some, like halite, dissolve easily in water.

Related terms: EvaporiteExample stones: Halite (rock salt), Fluorite

A mineral's resistance to being scratched. It is usually given on the Mohs scale, found by scratching minerals against each other or against everyday objects of known hardness such as a fingernail (about 2.5), a steel knife and glass (about 5.5). Hardness is not the same as resistance to breaking: diamond is very hard but can be split by a sharp blow.

Related terms: Mohs scale, Toughness, Brittleness, StreakExample stones: Talc, Quartz, Diamond

Also called: Heating

The controlled heating of gemstones to improve their colour or clarity, the oldest and most common gem treatment. Most rubies and sapphires are heated, most citrine on the market is heated amethyst, and tanzanite usually gets its blue-violet colour from heating. The result is permanent and widely accepted, but it should be disclosed.

Related terms: Gemstone treatment, IrradiationExample stones: Ruby, Sapphire, Citrine, Amethyst, Tanzanite

The crystal system with sixfold symmetry, described by three equal axes meeting at 120° in one plane and a fourth axis of a different length at right angles to them. The six-sided prisms of beryl (emerald, aquamarine) and apatite are typical examples.

Related terms: Crystal system, Trigonal systemExample stones: Beryl, Emerald, Aquamarine, Apatite

Describes processes and deposits involving hot, mineral-rich water solutions circulating through the Earth's crust. As these solutions cool or react with rocks, they deposit quartz, fluorite and baryte, as well as ore minerals such as galena, sphalerite, chalcopyrite and cinnabar, in cracks; many gold and metal deposits are hydrothermal.

Related terms: Vein, Ore deposit, Ore, GeodeExample stones: Quartz, Fluorite, Galena, Cinnabar

Also called: Magmatic rock

Rock formed when magma or lava cools and solidifies. Rock that cools slowly underground becomes coarse-grained plutonic rock (granite, gabbro), while rock that cools fast at the surface becomes fine-grained or glassy volcanic rock (basalt, obsidian).

Related terms: Magma, Plutonic rock, Volcanic rock, Rock cycleExample stones: Granite, Gabbro, Basalt, Obsidian

Also called: International Mineralogical Association

The International Mineralogical Association. Founded in 1958, its Commission on New Minerals, Nomenclature and Classification (CNMNC) decides whether a proposed new mineral is accepted and rules on mineral names; more than 6,000 valid mineral species are recognised today.

Related terms: Mineral, Type locality, Variety

Also called: Simulant

A natural or man-made material that looks like a gemstone but has a different chemical composition and different properties. Coloured glass and plastic, cubic zirconia and moissanite used in place of diamond, and plastics and resins passed off as amber are common imitations.

Related terms: Synthetic gemstone, Doublet, Triplet, DyeingExample stones: Diamond, Amber

A crystal of another mineral, a pocket of liquid, a gas bubble or a remnant of a living thing trapped inside a mineral or gem as it grew. Inclusions give clues to where and how a stone formed and help to tell natural stones from synthetic ones; insects in amber are among the best known.

Related terms: Synthetic gemstone, Asterism, Chatoyancy, Geographic originExample stones: Amber, Quartz, Emerald

Rainbow colours that change with the viewing angle, like those on a soap bubble, caused by the interference or diffraction of light in thin layers or regular structures. The sheen of pearl, the flash of labradorite and the play of colour of opal are forms of iridescence, and a thin tarnish film can make chalcopyrite iridescent too.

Related terms: Labradorescence, Play of colour, OpalescenceExample stones: Pearl, Labradorite, Opal, Chalcopyrite

A meteorite made mostly of iron–nickel alloy (kamacite and taenite), thought to come from the molten metal cores of ancient asteroids. It is very heavy, strongly attracted to a magnet, and its cut and acid-etched surface usually shows the Widmanstätten pattern. The largest known meteorite, Hoba in Namibia, weighing more than 60 tonnes, is an iron meteorite.

Related terms: Meteorite, Widmanstätten pattern, Pallasite, MagnetismExample stones: Iron meteorite

Exposing gemstones to radiation such as gamma rays, electrons or neutrons to change their colour, often followed by heating. Nearly all blue topaz on the market is irradiated colourless topaz, and some smoky quartz and coloured diamonds are irradiated too. Irradiated stones should be held back from sale until any residual radioactivity has fallen to a safe level.

Related terms: Gemstone treatment, Heat treatment, RadioactivityExample stones: Topaz, Smoky quartz, Diamond

Also called: Kimberlite pipe

A dark volcanic rock formed from magma that rose very rapidly from depths of more than 150 kilometres in the mantle, found in carrot-shaped pipes. Because it carries diamonds formed at depth up to the surface, it is the main primary source of diamond; it is named after the town of Kimberley in South Africa.

Related terms: Magma, Volcanic rock, Placer depositExample stones: Diamond

A vivid flash of colour, mostly blue and green but sometimes gold and orange, seen in labradorite from certain angles. It is caused by the interference of light in extremely thin parallel layers within the feldspar, and it is the hallmark of labradorite, which is named after Labrador in Canada.

Related terms: Iridescence, AdularescenceExample stones: Labradorite, Plagioclase

Magma that has reached the Earth's surface, and the rock formed when it cools and hardens. Because it cools quickly, its crystals stay small or never form: basalt mostly comes from lava flows, and obsidian from lava that froze before it could crystallise.

Related terms: Magma, Volcanic rock, Volcanic glassExample stones: Basalt, Obsidian, Pumice

Also called: Luster

The look of a mineral's surface, which depends on the way it reflects light. The main kinds are metallic (pyrite), vitreous or glassy (quartz), adamantine (diamond), resinous (amber), pearly (muscovite), silky (tiger's eye), greasy, waxy and dull (kaolinite).

Related terms: Vitreous lustre, Metallic lustre, Adamantine lustre, TransparencyExample stones: Pyrite, Quartz, Diamond, Amber, Muscovite, Tiger's eye, Kaolinite

A hot, mobile mixture of molten rock, dissolved gases and crystals that forms deep in the crust or in the upper mantle, at about 700–1,300 °C. Magma that cools slowly underground forms plutonic rocks, while magma that reaches the surface and flows out as lava forms volcanic rocks.

Related terms: Lava, Igneous rock, Plutonic rock, Volcanic rockExample stones: Granite, Basalt

The attraction of a mineral to a magnet, or its ability to act as a magnet itself. Magnetite is strongly magnetic, and some pieces (lodestone) can even hold iron filings; iron meteorites also stick to a magnet, which is why meteorite hunters carry one.

Related terms: Iron meteorite, MeteoriteExample stones: Magnetite, Iron meteorite

The ability of a mineral to be flattened and reshaped by hammering without breaking. Native gold, silver and copper are malleable, and gold can be beaten into leaf thin enough to let light through.

Related terms: Brittleness, Native element, ToughnessExample stones: Gold, Silver, Copper

Also called: Metallic luster

A strong, mirror-like lustre like that of polished metal, found in opaque minerals. Native gold, silver and copper and sulfides such as pyrite, galena and chalcopyrite are metallic; duller examples are called submetallic.

Related terms: Lustre, Sulfide, Native elementExample stones: Gold, Silver, Pyrite, Galena, Chalcopyrite

Rock formed when an existing rock is transformed into new minerals and textures by high temperature, pressure or hot fluids, without melting. Limestone becomes marble and sandstone becomes quartzite, while shale, as pressure and temperature rise, becomes slate, then schist and then gneiss.

Related terms: Metamorphism, Foliation, Rock cycleExample stones: Marble, Quartzite, Slate, Schist, Gneiss

The change in a rock's minerals and texture, while it stays solid, under the influence of heat, pressure and fluids. The baking of rocks around a hot body of magma is called contact metamorphism, and the burial and heating of large areas during mountain building is called regional metamorphism.

Related terms: Metamorphic rock, Foliation, Diagenesis, MagmaExample stones: Marble, Schist, Garnet

Also called: Shooting star

The streak of light seen in the sky when a meteoroid enters the atmosphere at high speed and glows as it heats up and vaporises by compressing the air ahead of it; popularly called a shooting star. A meteor is a flash of light, not a stone, and most meteors burn up completely before reaching the ground.

Related terms: Meteoroid, Meteorite, Fusion crust

A natural piece of rock or metal from space that survives its passage through the atmosphere and reaches the ground. Meteorites are grouped as stony (chondrites and achondrites), iron, and stony-iron (such as pallasites). The bright streak in the sky is the meteor; only the piece that lands is the meteorite.

Related terms: Meteoroid, Meteor, Chondrite, Iron meteorite, Pallasite, Fusion crust, TektiteExample stones: Chondrite, Iron meteorite, Pallasite

A small rocky or metallic body travelling through space, smaller than an asteroid; by the International Astronomical Union's definition it is roughly between 30 micrometres and 1 metre across. When it enters the atmosphere it is seen as a meteor, and any piece that reaches the ground is a meteorite.

Related terms: Meteor, Meteorite

A naturally occurring solid with a definite chemical composition and an ordered internal arrangement of atoms (a crystal structure). Quartz, calcite and gold are minerals; more than 6,000 mineral species are recognised today.

Related terms: Rock, Mineraloid, Crystal structure, Variety, IMAExample stones: Quartz, Calcite, Gold

A naturally occurring substance that resembles a mineral but is not counted as one, because it lacks an ordered crystal structure or a fixed composition. Opal, obsidian and amber are mineraloids.

Related terms: Mineral, Amorphous, Volcanic glassExample stones: Opal, Obsidian, Amber

Also called: Mohs hardness scale

A relative scale of hardness proposed by the German mineralogist Friedrich Mohs in 1812, based on the order in which ten minerals scratch one another: 1 talc, 2 gypsum, 3 calcite, 4 fluorite, 5 apatite, 6 orthoclase, 7 quartz, 8 topaz, 9 corundum, 10 diamond. The scale only ranks minerals; its steps are not equal, and diamond, for example, is far harder than corundum.

Related terms: HardnessExample stones: Talc, Gypsum, Calcite, Fluorite, Apatite, Orthoclase, Quartz, Topaz, Corundum, Diamond

The crystal system with three axes of unequal length, two of which meet at an oblique angle while the third is at right angles to both. Gypsum, orthoclase, micas such as muscovite and biotite, malachite and azurite are monoclinic.

Related terms: Crystal systemExample stones: Gypsum, Orthoclase, Muscovite, Malachite, Azurite

The mineral class made of elements that occur in nature on their own, or as natural alloys, rather than combined with other elements. Metals such as gold, silver and copper, sulfur, and the two forms of carbon, diamond and graphite, belong here.

Related terms: Malleability, Polymorph, Metallic lustreExample stones: Gold, Silver, Copper, Sulfur, Diamond, Graphite

Also called: Strunz classification

A classification that divides minerals into ten classes by chemical composition and crystal structure. It is based on tables first published by Karl Hugo Strunz in 1941, and its tenth edition (2001) was prepared with Ernest Nickel. Each mineral has a code such as 4.DA.05 (quartz); in this system quartz is placed among the oxides.

Related terms: Dana classification, IMA, Silicate, OxideExample stones: Quartz

Also called: Commercial onyx, Mexican onyx

Banded calcite or aragonite, deposited by springs or in caves, that is sold as “onyx” in the stone trade and in souvenir workshops. True onyx is a chalcedony (quartz) with straight black and white bands; onyx marble is much softer, can be scratched with a steel knife and fizzes in acid. Most of the “onyx” worked in the workshops of Cappadocia and Hacıbektaş in Türkiye is this stone.

Related terms: Commercial marble, Carbonate, DyeingExample stones: Onyx, Travertine, Calcite, Chalcedony

A milky, often bluish-white glow from within a stone, caused by the scattering of light. The name comes from opal, and it describes the cloudy look of common opal without play of colour, although the word is sometimes used loosely for play of colour too.

Related terms: Play of colour, Adularescence, IridescenceExample stones: Opal

A slice of oceanic crust and upper mantle that has been pushed up onto a continent by plate movements and exposed on land. From bottom to top it shows peridotite (often altered to serpentinite), gabbro and basalt with pillow lavas; ophiolites are widespread in Türkiye and host the country's main chromium (chromite) deposits.

Related terms: Ore deposit, MetamorphismExample stones: Peridotite, Serpentinite, Gabbro, Basalt, Chromite

A mineral or rock from which a metal or another useful substance can be extracted economically. Chalcopyrite is an ore of copper, galena of lead, sphalerite of zinc, hematite and magnetite of iron, and chromite of chromium; whether a rock counts as ore depends on its metal content and the cost of extracting it.

Related terms: Ore grade, Ore deposit, GangueExample stones: Chalcopyrite, Galena, Sphalerite, Hematite, Chromite

Also called: Mineral deposit

A place in the Earth's crust where one or more useful minerals have collected in enough quantity and concentration to be mined economically. Deposits are classified by how they formed, for example as magmatic, hydrothermal, sedimentary or placer deposits; the Guleman chromite deposits (Elazığ) and the Kırka borate deposit (Eskişehir) are examples from Türkiye.

Related terms: Ore, Ore grade, Hydrothermal, Placer deposit, OphioliteExample stones: Chromite, Borax (tincal)

Also called: Grade

The proportion of a valuable metal or mineral in an ore. It is usually given as a percentage for copper, lead and zinc and in grams per tonne (g/t) for gold and silver; the higher the grade, the less rock has to be processed for the same amount of metal.

Related terms: Ore, Ore deposit

Also called: Organic gemstone

Material produced by living things, or formed from their remains, that is used as a gemstone. Pearl, coral, amber (fossil resin) and Oltu stone (coalified wood) are leading examples; most are softer than mineral gems and sensitive to heat, acids and perfume.

Related terms: Gemstone, FossilExample stones: Pearl, Coral, Amber, Oltu stone

The crystal system with three axes at right angles to one another, all of different lengths. Topaz, olivine, baryte, sulfur and stibnite crystallise in this system.

Related terms: Crystal systemExample stones: Topaz, Olivine, Baryte, Sulfur, Stibnite

Also called: Oxide minerals

The mineral class formed by oxygen combined with one or more metals; hydroxides are usually grouped with it. Important ores such as hematite and magnetite (iron), chromite (chromium) and uraninite (uranium), and gems such as corundum (ruby, sapphire) and spinel, are oxides.

Related terms: Ore, Nickel–Strunz classificationExample stones: Hematite, Magnetite, Chromite, Corundum, Spinel

A rare stony-iron meteorite made of olivine crystals, often around a centimetre across, set in iron–nickel metal. When it is cut into thin polished slices, the golden-yellow and green olivine between the metal glows like stained glass; it is named after Peter Simon Pallas, who studied the first known example in the 18th century.

Related terms: Meteorite, Iron meteoriteExample stones: Pallasite, Olivine, Peridot

A very coarse-grained igneous rock whose crystals can grow from a few centimetres to several metres long; it is usually granitic in composition and occurs as veins or pockets. It forms from the last melt left in a magma, rich in water and rare elements, and is a major source of beryl (aquamarine), tourmaline, topaz and large mica crystals.

Related terms: Plutonic rock, Magma, VeinExample stones: Beryl, Aquamarine, Tourmaline, Topaz, Muscovite

Also called: Phosphate minerals

The mineral class containing the phosphate (PO₄) group. Its commonest member is apatite: the mineral part of our teeth and bones is an apatite-like material, and phosphate fertilisers are made from apatite-rich rock. Turquoise is a phosphate too.

Example stones: Apatite, Turquoise

Also called: Piezoelectric effect

The ability of some crystals to develop an electric charge on their surfaces when squeezed or stretched and, the other way round, to change shape very slightly when a voltage is applied. Thanks to it, quartz (today mostly synthetic quartz) keeps time and frequency in watches and electronic circuits; tourmaline is piezoelectric too.

Related terms: Synthetic gemstoneExample stones: Quartz, Tourmaline

Also called: Placer

A deposit formed where heavy, durable minerals are carried by rivers, waves or wind, separated from lighter material and concentrated in one place. Gold, diamond, sapphire, ruby and zircon collect this way in river gravels and on beaches; most of history's gold rushes were for placer gold.

Related terms: Erosion, Weathering, Specific gravity, Ore depositExample stones: Gold, Diamond, Sapphire, Zircon

Also called: Play-of-color

Bright patches of spectral colour that shift as the viewing angle changes, seen in precious opal. They are produced when light is diffracted by the tiny, regularly stacked silica spheres that make up the opal; opal without play of colour is called common opal.

Related terms: Opalescence, Iridescence, AmorphousExample stones: Opal

Also called: Dichroism, Trichroism

The property of some birefringent crystals of showing different colours or shades when viewed from different directions; two colours are called dichroism and three trichroism. Tanzanite looks blue or violet depending on the direction of view, and tourmaline is much darker when viewed along its crystal axis, so cutters orient a stone to show its best colour.

Related terms: Birefringence, CutExample stones: Tanzanite, Tourmaline

Also called: Intrusive rock

Igneous rock formed when magma cools slowly deep within the crust, over thousands to millions of years. Slow cooling gives the crystals time to grow, so the grains are visible to the naked eye; granite, diorite and gabbro are examples.

Related terms: Igneous rock, Magma, Volcanic rock, PegmatiteExample stones: Granite, Diorite, Gabbro

Also called: Polymorphism

Each of two or more minerals with the same chemical composition but different crystal structures. Diamond and graphite (carbon), calcite and aragonite (calcium carbonate), and kyanite, andalusite and sillimanite (aluminium silicate) are polymorphs; temperature and pressure decide which one forms.

Related terms: Crystal structure, Mineral, Solid solutionExample stones: Diamond, Graphite, Calcite, Kyanite

An igneous rock texture in which large, visible crystals (phenocrysts) are scattered through a fine-grained or glassy groundmass. It shows that the magma cooled in two stages: the large crystals grew slowly at depth and the groundmass formed quickly after the magma rose. It is common in andesite and dacite.

Related terms: Volcanic rock, Igneous rock, MagmaExample stones: Andesite, Dacite

Rock formed when ash, pumice and rock fragments blasted out by explosive volcanic eruptions settle and become consolidated. Tuff and ignimbrite belong to this group; the fairy chimneys of Cappadocia were carved by erosion from thick layers of tuff and ignimbrite.

Related terms: Volcanic rock, Erosion, Volcanic glassExample stones: Tuff, Ignimbrite, Pumice

An open working from which building and ornamental stone, crushed rock, or raw material for lime and cement is extracted. Türkiye is known for its marble and travertine quarries, and those on Marmara Island have been worked since antiquity. Places where metal ores or coal are dug are usually called mines.

Related terms: Commercial marble, Commercial granite, Ore depositExample stones: Marble, Travertine, Limestone

The spontaneous decay of some atomic nuclei, giving off radiation. Minerals containing uranium and thorium are radioactive: uraninite strongly so and some zircon weakly, and the steady rate of this decay is used to date rocks. Radioactive minerals should be stored away from living spaces and their dust should not be breathed in.

Related terms: IrradiationExample stones: Uraninite, Zircon

Also called: RI

A number showing how much light slows down in a material compared with air, and so how strongly it is bent. Gemmologists identify many stones by measuring it with a refractometer: quartz has a refractive index of about 1.54–1.55 and diamond 2.42, and a high index gives a stone a strong lustre.

Related terms: Birefringence, Dispersion, Adamantine lustreExample stones: Quartz, Diamond

A solid, naturally formed mass made of one or more minerals (sometimes also of volcanic glass or organic matter). Granite is made of quartz, feldspar and mica, while limestone is almost entirely calcite. Rocks are grouped by origin as igneous, sedimentary or metamorphic.

Related terms: Mineral, Igneous rock, Sedimentary rock, Metamorphic rock, Rock cycleExample stones: Granite, Limestone, Obsidian

A model of how rocks are turned into one another over millions of years. Igneous rocks are weathered and eroded into the sediment that makes sedimentary rocks, buried rocks are changed by heat and pressure into metamorphic rocks, and rocks that melt at depth become magma again, so the cycle goes on.

Related terms: Igneous rock, Sedimentary rock, Metamorphic rock, Weathering, Erosion, MagmaExample stones: Granite, Sandstone, Quartzite

Rock formed when sediments laid down at the Earth's surface (sand, mud, gravel, shells, plant remains or minerals that settle out of water) are compacted and cemented into stone. Sandstone, shale and conglomerate are clastic; limestone, travertine and rock salt are chemical or biochemical; and coal is an organic sedimentary rock. Most fossils are found in these rocks.

Related terms: Diagenesis, Evaporite, Fossil, Rock cycle, WeatheringExample stones: Sandstone, Shale, Conglomerate, Limestone, Coal

A traditional name for gemstones other than diamond, ruby, sapphire and emerald. It has no scientific basis: some “semi-precious” stones, such as a top-quality tourmaline, can be worth far more than an ordinary ruby, and CIBJO, the World Jewellery Confederation, regards the term as misleading.

Related terms: GemstoneExample stones: Tourmaline, Ruby

Also called: Silicate minerals

The mineral class whose basic building block is the SiO₄ tetrahedron, one silicon atom bonded to four oxygen atoms; it is the largest mineral group and makes up most of the Earth's crust. Silicates are subdivided by how the tetrahedra are linked (isolated, in rings, chains, sheets or three-dimensional frameworks), giving groups such as olivine, beryl, augite, the micas and the feldspars.

Related terms: Mineral, Nickel–Strunz classification, Dana classificationExample stones: Olivine, Beryl, Augite (pyroxene), Muscovite, Orthoclase

An incurable occupational lung disease caused by breathing in fine dust containing crystalline silica, such as quartz, over a long period. It occurs in mining, quarrying, stone cutting, sandblasting and the fabrication of engineered quartz worktops; in Türkiye, cases among denim sandblasters led to a ban on the practice in 2009. It is prevented by wet working, ventilation and suitable respirators.

Related terms: Asbestos, QuarryExample stones: Quartz, Chert and flint, Sandstone

Also called: Isomorphous series, Isomorphism

A mineral series in which some atoms in the crystal structure can be swapped for other atoms of similar size; the end members of the series share the same structure (they are isomorphous). In olivine, magnesium and iron replace each other, and plagioclase forms a continuous series from sodium-rich albite to calcium-rich anorthite.

Related terms: Mineral, Crystal structure, PolymorphExample stones: Olivine, Plagioclase, Garnet

Also called: Relative density, SG

The ratio of a mineral's weight to the weight of an equal volume of water. It has no unit and is numerically almost the same as the density in g/cm³. You can feel it in the hand: quartz has a specific gravity of about 2.65, galena about 7.5 and pure gold 19.3.

Related terms: Density, Placer depositExample stones: Quartz, Galena, Gold

The colour of the powder a mineral leaves when rubbed across an unglazed white porcelain tile (a streak plate). It can differ from the mineral's own colour and is more reliable for identification: black or silvery hematite leaves a red-brown streak, brassy yellow pyrite a greenish-black one. Minerals harder than the plate (above about 7) leave no streak.

Related terms: Hardness, LustreExample stones: Hematite, Pyrite

Also called: Sulphate, Sulfate minerals

The mineral class containing the sulfate (SO₄) group. Gypsum, baryte and celestine are well-known members; gypsum is used to make plaster and cement, and baryte, because of its weight, in drilling mud.

Related terms: EvaporiteExample stones: Gypsum, Baryte, Celestine

Also called: Sulphide, Sulfide minerals

The mineral class formed by sulfur combined with one or more metals. Most sulfides are heavy and have a metallic lustre, and the class includes key ore minerals such as pyrite, galena (lead), sphalerite (zinc), chalcopyrite (copper), cinnabar (mercury) and stibnite (antimony).

Related terms: Ore, Metallic lustre, HydrothermalExample stones: Pyrite, Galena, Sphalerite, Chalcopyrite, Cinnabar, Stibnite

Also called: Lab-grown gemstone, Lab-created gemstone

A stone made in a laboratory that has the same chemical composition, crystal structure and properties as its natural counterpart. Synthetic ruby has been made by flame fusion (the Verneuil process) since the early 1900s, and synthetic diamond is grown by high-pressure, high-temperature (HPHT) or chemical vapour deposition (CVD) methods. A synthetic is not an imitation, but selling it as natural is fraud.

Related terms: Imitation, Inclusion, Gemstone, Gemstone treatmentExample stones: Ruby, Diamond, Quartz

Natural glass formed when Earth rocks were melted by a large meteorite impact, flung into the air and cooled quickly. Because it is made of Earth material, it is not a meteorite; the green moldavite of Central Europe is the best-known kind.

Related terms: Meteorite, Volcanic glass, AmorphousExample stones: Tektite

The crystal system with three axes at right angles, two of equal length and the third longer or shorter. The square-sectioned prisms and pyramids of zircon are a typical example; chalcopyrite is tetragonal too.

Related terms: Crystal systemExample stones: Zircon, Chalcopyrite

A stone's resistance to breaking, cracking and chipping, which is a different property from hardness. Jadeite and nephrite (jade), made of tightly interlocking crystals or felted fibres, are very tough, while diamond, though extremely hard, can be split along its cleavage by a sharp blow.

Related terms: Hardness, Brittleness, CleavageExample stones: Jade, Diamond

Also called: Diaphaneity

How much light a mineral lets through. Transparent minerals show objects behind them clearly (rock crystal), translucent ones let light through but blur the image (chalcedony), and opaque minerals let no light through (pyrite).

Related terms: LustreExample stones: Quartz, Chalcedony, Pyrite

The crystal system with the lowest symmetry: three axes of different lengths, none of them at right angles to another. The plagioclase feldspars, kyanite, turquoise and ulexite are triclinic.

Related terms: Crystal systemExample stones: Plagioclase, Kyanite, Turquoise, Ulexite

Also called: Rhombohedral system

A crystal system that uses the same set of axes as the hexagonal system but has a threefold rather than a sixfold axis of symmetry; some sources treat it as a subdivision of the hexagonal system. Quartz, calcite, corundum (ruby, sapphire), tourmaline and hematite are trigonal.

Related terms: Crystal system, Hexagonal systemExample stones: Quartz, Calcite, Corundum, Tourmaline, Hematite

A composite stone made of three layers glued together. The most common is the opal triplet, in which a thin slice of opal sits between a dark backing and a clear protective dome of quartz or glass. The layers can be seen from the side, and water can damage the glue.

Related terms: Doublet, ImitationExample stones: Opal

Also called: Twin crystal

The growth of two or more crystals of the same mineral joined together according to a definite symmetry rule. The swallowtail twins of gypsum, the Carlsbad twins of orthoclase and the repeated twinning of plagioclase, seen as fine parallel lines (striations) on its surfaces, are familiar examples.

Related terms: Crystal, Crystal habitExample stones: Gypsum, Orthoclase, Plagioclase

The place where a mineral species was first found and scientifically described; the mineral's official description is based on specimens (type material) from there. Many minerals are named after their type locality: aragonite, for example, takes its name from Molina de Aragón in Spain.

Related terms: IMA, Mineral

A form of a mineral species that is set apart by features such as colour, transparency or pattern and has its own name; it is not a separate species. Amethyst, citrine and smoky quartz are varieties of quartz, ruby and sapphire of corundum, and emerald and aquamarine of beryl.

Related terms: Mineral, IMAExample stones: Amethyst, Citrine, Smoky quartz, Ruby, Sapphire, Emerald

A sheet-like body of minerals formed when a crack in rock is filled by minerals deposited from solutions passing through it. White quartz veins are common, and gold, silver and many metal ores are mined from hydrothermal veins.

Related terms: Hydrothermal, Ore deposit, Pegmatite, GangueExample stones: Quartz, Gold, Silver

Also called: Vitreous luster, Glassy lustre

A lustre like that of broken or polished glass, and the most common lustre in minerals. Quartz, beryl, tourmaline and calcite have a vitreous lustre.

Related terms: LustreExample stones: Quartz, Beryl, Tourmaline, Calcite

Amorphous natural glass formed when lava cools too quickly for crystals to grow. Obsidian is dense volcanic glass and pumice a frothy one full of gas bubbles; because the glass slowly crystallises over time, very old volcanic glass is rare. Tektites are natural glass too, but they were formed by meteorite impacts, not volcanoes.

Related terms: Amorphous, Lava, Tektite, Conchoidal fractureExample stones: Obsidian, Pumice, Tektite

Also called: Extrusive rock

Igneous rock formed when lava or material thrown out by volcanic eruptions cools quickly at or near the surface. Its crystals are too small to see easily with the naked eye, or absent altogether; basalt, andesite, rhyolite, obsidian and pumice belong to this group.

Related terms: Igneous rock, Lava, Plutonic rock, Porphyritic texture, Pyroclastic rockExample stones: Basalt, Andesite, Rhyolite, Obsidian, Pumice

The breaking down and decay of rocks in place, at or near the Earth's surface, by water, air, temperature changes and living things. In physical weathering the rock is broken apart, for example by water freezing in cracks; in chemical weathering the minerals change, as when feldspar turns into the clay mineral kaolinite. The removal of the weathered material is called erosion.

Related terms: Erosion, Rock cycle, Sedimentary rock, Placer depositExample stones: Kaolinite, Orthoclase, Granite

Also called: Widmanstätten structure, Thomson structure

A pattern of thin, criss-crossing metal bands seen on the cut, acid-etched surface of iron meteorites. It forms as two iron–nickel alloys, kamacite and taenite, separate during extremely slow cooling over millions of years in asteroid cores, which makes it strong evidence of a meteoritic origin.

Related terms: Iron meteorite, MeteoriteExample stones: Iron meteorite