Mineralogy and lithotherapy glossary: technical terms explained

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How to read this glossary

This glossary explains in simple words the terms you will come across in stone descriptions, in the profiles of our guides and in the world of lithotherapy. Terms are listed in alphabetical order within each part. An italicized word or “see” points to another entry.

A useful reminder: lithotherapy rests on traditions and symbols. The definitions on this page distinguish what can be measured (hardness, luster, composition) from what belongs to symbolism.

Letter A

• Adularescence: A bluish or milky white sheen that seems to float beneath the surface of certain stones when they are moved, especially moonstone (a feldspar). It comes from the stacking of very thin lamellae inside the crystal.
• Aggregate: An assembly of small crystals or grains fused together, without a regular overall shape. Many opaque or banded stones are aggregates.
• Amorphous: Describes a material whose atoms are not arranged in a regular order, and which therefore has no crystal structure. Opal and glass are amorphous.
• Asterism: A four-, six- or twelve-rayed star effect that appears on a cabochon-cut stone, for example a sapphire or a “star” rose quartz. It is caused by fine needles (often rutile) oriented within the stone.
• Aventurescence: A glitter of tiny flakes that reflect light inside a stone: this is the effect of aventurine and sunstone. Goldstone reproduces the look with copper crystals in glass.

Terms, part 2

• Birefringence: The property of certain crystals to split a ray of light into two. It produces a double image in clear calcite, and gemologists use it to identify stones.
• Cabochon: A stone cut with a smooth, domed surface and no facets, often oval or round. It is the usual cut for opaque or translucent stones (turquoise, lapis lazuli, tiger's eye).
• Carat: The unit of mass for precious stones: 1 carat equals 0.2 gram. Do not confuse it with the “carat” of gold, which measures its purity (24 carats for pure gold, 18 carats for 75% gold).
• Chakra: A Sanskrit word meaning “wheel”. In Indian traditions, these are energy centres of the body. In lithotherapy, stones are associated with each chakra, mainly by colour. This is a symbolic correspondence, not scientifically demonstrated.
• Chatoyancy: A moving bright band that glides across the surface of a cabochon-cut stone when it is turned, like a cat's eye. It is caused by parallel fibres or needles. Examples: tiger's eye, cat's eye (chrysoberyl).
• Cleavage: The tendency of certain minerals to split along smooth, very precise planes. Mica separates into sheets, fluorite into octahedra. Perfect cleavage makes a stone sensitive to knocks, even if it is hard (topaz, moonstone).
• Cryptocrystalline: Formed of crystals so small they cannot be seen, even with a magnifier. Chalcedony, agate and jasper are cryptocrystalline.
• Crystal: Several meanings. In science: a solid whose atoms are arranged in a regular order, hence flat faces and precise angles. In everyday language: a rock crystal, or a fine crystal of any mineral. In tableware: a glass containing lead oxide, which has nothing to do with a mineral.
• Crystalline: Formed of crystals, or having the clarity of a crystal. A crystalline rock is made of crystals, visible or not to the naked eye. Not to be confused with the crystalline lens of the eye.
• Crystallisation: The process by which atoms arrange themselves into crystals, through the cooling of molten rock, the evaporation of salt-laden water, or deposition from a hot fluid in the fissures of a rock.
• Fracture: The way a mineral breaks when it does not follow a cleavage plane. It may be conchoidal (rounded, like broken glass), splintery, uneven or flat. It is a clue for identification.

Terms, part 3

• Dendrite: A branching black or brown pattern resembling small plants, formed by manganese or iron oxides in a crack or on the surface of a stone. It is not a plant fossil. It is seen in dendritic agate.
• Density: The ratio between the mass of a material and that of the same volume of water. It is an identification measure: quartz has a density of about 2.65, diamond about 3.5.
• Dichroism, trichroism, pleochroism: The property of certain crystals to show two colours (dichroism), three colours (trichroism) or several (pleochroism) depending on the angle from which they are viewed. Tanzanite, iolite and tourmaline are good examples.
• Diffusion (treatment by): A treatment that drives colouring chemical elements into the stone at high temperature. It has been used on sapphires. It must be disclosed.
• Doublet: A stone made of two pieces glued together, for example a thin slice of opal on a dark backing. It is not a single whole stone: it must be disclosed as an assembly. See also triplet.
• Druse: A surface or crust covered with small, tightly packed crystals, often inside a cavity. Quartz, calcite or agate druse is common.
• Enhydros: A crystal or nodule that contains a trapped bubble of water (and air), visible when the stone is tilted. Enhydros quartz and chalcedony exist.
• Hardness: A mineral's resistance to scratching, measured by comparison on the Mohs scale. Hardness is not toughness: a hard mineral can break if it has cleavage. See Mohs scale and toughness.
• Luster: The appearance of a mineral's surface in light. We speak of vitreous luster (quartz), pearly (selenite), metallic (pyrite), greasy (certain serpentines), resinous (amber), silky (tiger's eye), adamantine (diamond) or dull. Also spelled lustre.
• Mineral species: A recognised mineral, distinct because it has a well-defined chemical composition and structure. Quartz is a species; amethyst is one of its varieties. An international association maintains the list of species.
• Mohs scale: A hardness scale from 1 to 10, devised by the mineralogist Friedrich Mohs (1812): talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10. A mineral scratches those lower than itself. The scale is relative: the hardness gap between 9 and 10 is much larger than between 1 and 2.

Terms, part 4

• Deposit: A place where a mineral or rock is found in exploitable quantity, at the surface or at depth.
• Facet: A small flat, polished surface on a cut stone. Facets reflect light and create the stone's sparkle.
• Faceted (stone): A stone cut with facets to enhance its brilliance: this is the cut of transparent stones (diamond, sapphire, amethyst). Opaque stones are more often cut as cabochons.
• Feldspar: A group of very common minerals in the Earth's crust, which includes moonstone, amazonite and labradorite.
• Fluorescence: A glow that some minerals give off under ultraviolet light. It helps identification (the word comes from fluorite). It is not a criterion of quality or of “virtue”.
• Gangue: Worthless rock or mineral that surrounds the sought-after mineral in a deposit.
• Gem: A natural material chosen for its beauty, rarity and durability, for jewellery or an art object. The term covers minerals, but also organic materials such as pearl or amber.
• Gemology: The science and technique of studying gems: identification, quality, treatments, origin. Gemological laboratories issue certificates.
• Geode: A rounded cavity in a rock, lined inside with crystals (quartz, amethyst, calcite…). It is most often sold cut in half to show the interior.
• Habit: The usual shape of a mineral's crystals: needles, prisms, cubes, lamellae… Quartz forms prisms ending in a point; pyrite often forms cubes.
• Hydrophane: Describes an opal that absorbs water: it becomes more transparent when wet and regains its appearance as it dries. Many Ethiopian opals are hydrophane.
• Mineral family: A group of minerals of similar chemical composition: silicates, carbonates, oxides, sulfides, phosphates, sulfates… Silicates (quartz, beryl, tourmaline…) are the most numerous.

Terms, part 5

• Imitation: A material that resembles a stone but is of another nature: glass, plastic, or another mineral that has been dyed. It differs from a synthetic stone, which has the same composition as the natural stone.
• Impregnation: Filling pores or fissures with an oil, wax or colourless resin to improve appearance. Very common for emerald and turquoise. It must be mentioned.
• Inclusion: A foreign body (crystal, bubble, liquid, fissure) trapped in a stone when it formed. Inclusions tell the story of the stone and help distinguish natural from synthetic; they are not always a flaw.
• Iridescence: A rainbow play of colour that changes with the viewing angle, produced by the interference of light in very thin layers or fissures. It is seen on “rainbow” quartz, or on “aura quartz”. Not to be confused with the play of colour of opal, or with labradorescence.
• Irradiation: Exposure of a stone to radiation to change its colour. It is used for blue topaz or certain coloured diamonds. Irradiated stones offered for sale are checked in accordance with the regulations. The treatment must be disclosed.
• Labradorescence: Blue, green, golden or violet reflections, visible only from certain angles, on labradorite. They are caused by fine feldspar lamellae.
• Lapidary: An artisan who cuts and polishes stones; by extension, the art of cutting them. In the Middle Ages, a “lapidary” was also a book about stones and their supposed virtues.
• Lithotherapy: The practice of wearing or keeping stones close for well-being, based on traditions (also called crystal therapy). It is classed among non-conventional medicines, and no scientific evidence of its effectiveness exists: studies show no effect beyond placebo. Stones are inert and harmless in themselves, but lithotherapy must never replace medical advice or treatment.
• Play of colour: Bright, shifting flashes of colour that move across the surface of a precious opal when it is tilted. They come from tiny stacked spheres of silica that split light.
• Refractive index: A number that measures how much light is slowed and bent when it enters a material. The higher it is, the more brilliant the stone can be. It is a basic tool of the gemologist.

Terms, part 6

• Macle (twinning): A regular association of two or more crystals of the same species, growing into one another according to a precise symmetry (the crystals are said to be “twinned”). Cross-shaped staurolite and “Japan law” quartz are examples.
• Matrix: The rock or mineral in which a crystal is embedded. For turquoise, the “matrix” refers to the veinlets of rock that run through it.
• Mineral: A natural solid substance with a defined chemical composition and an ordered crystal structure. Quartz, calcite and beryl are minerals.
• Mineralogy: The science that studies minerals: their composition, structure, formation and properties.
• Nacre: The shiny layer that lines the inside of certain shells, produced by the mollusk. It is made mainly of aragonite. A pearl is formed of layers of nacre.
• Natural (stone): A stone extracted from the earth and simply cut and polished, without any treatment that changes its appearance. A “natural” stone may nonetheless be treated: always ask.
• Opalescence: A diffuse, slightly bluish milky glow in certain so-called common opals. It differs from the play of colour.
• Orgonite: A manufactured object combining resin, metal shavings and often crystals (quartz). It draws on the theories of Wilhelm Reich about an “orgone energy”, a notion that science has not confirmed. It is therefore a decorative and symbolic object.

Terms, part 7

• Birthstone: A stone associated, by custom, with a birth month. Several lists exist depending on country and era; see our pages on birthstones.
• Cut stone: A stone whose shape has been designed by a lapidary: cabochon, facets, bead, engraving, cameo…
• Fine stone: A trade term, now less used, for ornamental or jewellery stones that are not “precious stones”. The term “semi-precious” was also used: the expression is misleading, since some of these stones are rarer than the precious ones.
• Piezoelectricity: The property of certain crystals, including quartz, to produce a small electric voltage when compressed, and conversely to vibrate under a voltage. It is used in quartz watches.
• Polished stone: A stone whose surface has been smoothed and made shiny by rubbing with increasingly fine abrasives, by hand or by machine. The shape may stay irregular or be worked (sphere, egg, point).
• Porosity: The presence of tiny voids in a material. A porous stone (turquoise, lapis lazuli, certain agates) absorbs water, oils and dyes; it requires gentler care and may be dyed or stabilised.
• Precious stone: A historical and commercial term: diamond, ruby, sapphire and emerald. This classification is not based on science, but on rarity, beauty and hardness.
• Pseudomorph: A mineral that has taken the form of another by replacing it, atom by atom. For example, pyrite cubes transformed into goethite (an iron oxide) keep the original cubic shape.
• Purification (lithotherapy): A symbolic gesture of “cleansing” stones before wearing them: water, smoke, moonlight… It has no scientific value. Some gestures damage stones: see our guide to purifying your stones.
• Purity: The degree to which a stone is free of visible inclusions and flaws. For diamond, it is one of the four criteria (along with weight, colour and cut).
• Rough stone: A stone as it was extracted, neither cut nor polished, with its natural shape and surface. It may have sharp edges.
• Tumbled stone: A small, rounded, smooth stone obtained by tumbling: fragments of stone are placed in a drum with abrasives and turn for days or weeks. It is affordable and easy to handle.

Terms, part 8

• Crystal system: A family of crystal shapes, according to their symmetry. There are seven: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic (six if hexagonal and trigonal are grouped together).
• Quartz: A very common mineral made of silica (silicon dioxide), with a hardness of 7. It occurs in many varieties: rock crystal, amethyst, citrine, rose quartz, smoky quartz, tiger's eye…
• Reconstitution (reconstituted stone): A material obtained by binding powder or small fragments of stone with resin or glue. Reconstituted turquoise and reconstituted amber (called “ambroid”) exist. It is a manufactured product, and must be disclosed.
• Refraction: The change in direction of a ray of light as it passes from one medium to another, for example from air into a stone. See refractive index.
• Rock: A natural assemblage of minerals, or of non-crystalline matter, forming part of the Earth's crust: granite, limestone, obsidian… A rock therefore does not have a single hardness.
• Setting: Fixing a stone in the metal of a piece of jewellery, with prongs, a closed bezel or a rail. A good setting protects the stone and holds it firmly.
• Stabilisation: A treatment that impregnates a porous or friable stone with colourless resin to harden it and fix its colour. Common for turquoise. See impregnation.
• Synthetic: Made in a laboratory, with the same chemical composition and structure as the natural stone (synthetic ruby, sapphire, emerald, diamond). It is not an imitation, but neither is it a natural stone: the seller must say so.

Terms, part 9

• Cut: The way a stone is cut and polished: its overall shape (round, oval, pear…) and the type of facets or cabochon. A fine cut enhances colour and brilliance.
• Glass: An amorphous material made by melting sand and other ingredients. It is used as an imitation of stones. There are also natural glasses, such as obsidian, formed from lava.
• Heat treatment: Controlled heating of a stone to change its colour or clarity: sapphires, aquamarines, tanzanites, citrines (by heating amethyst). Often stable and common, but must be disclosed.
• Hue and dye: The hue is the shade of a colour. A dyed (or artificially coloured) stone has received a colouring agent: blue or pink agate, blue howlite, dyed jade. This treatment is common on chalcedonies, but must be disclosed.
• Quenching: Several meanings. In glassmaking or metallurgy, rapid cooling to harden. For stones, it is the act of plunging a heated stone into cold water to crack it: this is how “crackle quartz” is made, often dyed afterwards.
• Streak: The colour of the powder left when a mineral is rubbed on unglazed porcelain. It is often different from the colour of the mineral: grey hematite leaves a reddish-brown streak.
• Toughness: A mineral's resistance to impact and breakage. It is independent of hardness: jade is very tough despite a moderate hardness, whereas diamond, very hard, can split.
• Transparency: A material's ability to let light through: a stone is transparent (one can see clearly through it), translucent (light passes through, but the image is blurred) or opaque.
• Triplet: A stone made of three pieces glued together, for example a thin slice of stone, a coloured layer and a crystal top. Opal and emerald triplets exist. It must be disclosed as an assembly.
• Turquenite: Trade name for howlite or magnesite dyed blue to imitate turquoise. It is a common imitation.
• Ultrasonic cleaning: A bath that cleans jewellery by means of vibrations. It suits hard, sound stones, but damages cracked, impregnated or fragile stones: emerald, opal, pearl, turquoise, tanzanite…
• Variety: A particular form of a mineral species, distinguished by its colour or appearance. Amethyst, citrine and rock crystal are varieties of quartz.
• Zoning: The distribution of colour in bands or zones within a crystal. It can be seen in watermelon tourmaline (pink at the core, green outside) and in ametrine (amethyst-citrine).

The Mohs scale at a glance

Benchmarks to help you place a stone. Mohs hardness is compared by scratching: a mineral scratches those that are lower on the scale.

Hardness	Reference mineral	Everyday benchmark
1	Talc	Scratched by a fingernail, very soft
2	Gypsum (selenite)	Scratched by a fingernail
3	Calcite	Same hardness as a copper blade
4	Fluorite	Scratched by a steel knife
5	Apatite	Scratched by a knife, with a little effort
6	Orthoclase (feldspar)	Barely scratches ordinary glass
7	Quartz	Scratches glass; withstands daily wear
8	Topaz	Very hard
9	Corundum (ruby, sapphire)	Almost as hard as diamond
10	Diamond	The hardest known mineral

These everyday benchmarks (fingernail, coin, knife) are approximate. Avoid scratching your stones to “test” them: you would damage them. For the hardness of the stones we describe, see the stone lexicon.

Going further

To apply this vocabulary to your own stones, read our guide to choosing and caring for your stones, which explains how to recognise a natural, treated or manufactured stone. For next steps, the guide to purifying and recharging your stones describes traditional purification gestures and their material limits.

If a word is not listed here, do not hesitate to ask us: we will complete this glossary.

Frequently asked questions

Also read

The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Guides.