Mineral Glossary

Needle-like crystals, long and very thin — think of natrolite, rutile or millerite sprays. Fragile; handle by the matrix only.

Cleaned with acids, for example oxalic acid to remove iron stains or hydrochloric acid to remove calcite. Done well it is accepted, but it must be said.

The very bright, almost diamond-like shine of minerals with a high refractive index, such as cerussite, anglesite and diamond.

A horizontal tunnel driven into a hillside to reach or drain a mine.

The soft, floating bluish sheen of moonstone, caused by light scattering between microscopic layers inside the feldspar.

How beautiful and balanced a specimen looks: colour, lustre, contrast, composition and display. Often the first factor in price.

Any group of crystals or grains grown together. Clusters, sprays, rosettes and druses are all types of aggregate.

A mineral whose colour comes from traces of other elements or from defects, not from its own formula. Quartz is colourless when pure; amethyst, citrine and smoky are allochromatic.

Minerals formed in Alpine clefts: quartz, adularia, fluorite, titanite, epidote and the classic "Alpine" species. Refers to the geological process, so it applies to the Alps and to similar settings worldwide.

A gas bubble in volcanic rock later filled with minerals. The zeolites of India and many agates come from amygdales in basalt.

A crystal without its own flat faces, because it grew squeezed between other grains. The opposite of euhedral.

Pieces from different specimens glued together to look like one natural piece (a marriage). It must always be declared and lowers the value a lot.

A star-shaped reflection seen on some cabochon-cut stones — star sapphire, star ruby, star garnet — produced by oriented needle inclusions.

A glittering effect from tiny reflective inclusions, as in sunstone and aventurine.

A crystal with two clearly separate colours, as in many tourmalines, ametrine and some fluorite.

Two pyramids joined base to base, giving a crystal pointed at both ends. Typical of anatase, scheelite and sulphur.

Flattened, elongated crystals like knife blades. Kyanite, stibnite and some baryte grow this way.

A rounded, grape-like surface formed by radiating crystals growing outward from many centres. Characteristic of malachite, hematite, chrysocolla and smithsonite.

A specimen made to be displayed in a cabinet, over about 8 cm. Our Size filter divides them into Medium Cabinet (8-15 cm), Large Cabinet (15-25 cm) and Museum Size (25+ cm).

A stone cut with a smooth domed top and no facets, often used for opal, chatoyant and star stones.

A rare igneous rock made mostly of carbonate minerals, a source of unusual species. Kovdor (Russia) and Oka (Canada) are famous examples.

A penetration twin of two crystals grown through each other, classic for orthoclase feldspar. It is named after Karlovy Vary (Carlsbad) in the Czech Republic.

The number a collector or museum gives a specimen, often painted on it or written on the label. It helps trace its history.

A document that states what a specimen is, where it comes from and who sells it. Every ITALMINERAL specimen comes with one.

The "cat's eye" effect: a single band of light that moves across the surface, caused by parallel fibrous inclusions or structure. Tiger's eye is the classic example.

A small missing piece on an edge or a termination.

A historic locality famous for a mineral, such as Elba for pyrite or Tsumeb for its secondary minerals. Pieces from classic localities keep their value.

The tendency to split along flat planes. Fluorite, calcite and mica cleave easily, so they need careful handling.

An open crack in rock, lined with crystals that grew from circulating fluids. Alpine clefts are the classic source of crystal-clear quartz and adularia — and the goal of the Strahler.

Several crystals grown together as a single intergrown group, as opposed to one isolated crystal.

Quartz coated with a thin metal film in a lab to create rainbow colours. It is a treatment, not a natural colour.

A crested aggregate of tabular crystals that looks like a rooster's comb. It is typical of marcasite and baryte.

A specimen good enough for a serious collection: natural, well crystallised, in good condition and with a clear locality.

A defect in the crystal lattice that absorbs light and gives colour, often created by natural radiation. Smoky quartz and purple fluorite owe their colour to colour centres.

A mineral that changes colour under different light, for example from daylight to warm lamp light. Alexandrite is the famous example.

Bands or zones of different colour inside one crystal, recording changes during growth. Common in fluorite, amethyst and tourmaline.

Long, parallel, column-like crystals packed side by side, as in some calcite, aragonite or tourmaline.

A specimen where two or more well-crystallized species share the matrix in an attractive arrangement. Valued above the sum of its parts when the contrast is strong.

A hard, rounded mass that forms inside sedimentary rock around a nucleus, often a fossil or a grain of sand.

The area where a specimen was joined to surrounding rock or another crystal before removal. A natural feature, but one that should always be disclosed.

Two or more crystals growing through or across each other at an angle, without being a true twin. Crossed actinolite, rutile or stibnite are much loved by collectors.

The seven families of crystal symmetry: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic.

Crystals shaped like a cube, with six square faces. Think of pyrite from Navajún, fluorite or galena.

Three, four or six crystals twinned in a ring, often looking hexagonal. Examples are chrysoberyl trillings, rutile sixlings and aragonite.

The loss of water from some minerals in dry air, which can turn them white or crumbly. Borax and some zeolites can dehydrate.

Branching, tree- or fern-like growth, usually of manganese oxides on a rock surface or of native copper and silver. Often mistaken for fossil plants.

Small, brilliant, doubly terminated quartz crystals of the Herkimer type, found in several localities worldwide. Gem-like sparkle in a floater form.

How much light passes through a mineral: transparent, translucent or opaque.

Collector slang for a small bruise or chip on a crystal edge or face.

The splitting of white light into rainbow flashes (fire). Diamond, sphalerite, titanite and cerussite show strong dispersion.

A small stand, often acrylic, that holds a specimen in its best position.

The best-looking side of a specimen, chosen for display and photos.

A crystal with twelve faces. The rhombic dodecahedron is the classic garnet shape.

Light splits into two rays inside the crystal, so you see a double image through it. Optical calcite (Iceland spar) shows it clearly.

A crystal with natural terminations at both ends, meaning it grew without being attached to a surface. Typically more desirable than a single-terminated crystal of the same species.

A carpet of tiny crystals covering a surface, often lining a cavity. Adds sparkle and texture rather than individual crystal definition.

The pile of waste rock outside a mine. Many collectors still find good minerals on old dumps.

Coloured artificially with dyes, as is common with agate. Natural colours are always preferred.

Also called a geniculate twin: two crystals joined at an angle, like a bent elbow. It is typical of rutile and cassiterite.

A crust of one mineral that formed over another, which later dissolved away — leaving a hollow shell preserving the original crystal's outline.

The oriented growth of one mineral on another, guided by the crystal lattice underneath. Golden rutile stars on hematite are a famous example.

A crystal whose faces were partly dissolved by later fluids, leaving pits, grooves or a frosted texture. Natural, and sometimes very beautiful — but always disclosed.

A crystal bounded by its own well-formed flat faces. Collectors look for sharp, euhedral crystals.

Minerals left behind when salty water evaporates, such as halite, gypsum and sylvite.

A specimen that previously belonged to a documented, usually notable, collection. Provenance of this kind adds both historical interest and value.

Flat quartz crystals crossed by a white thread-like line (Faden means thread in German), formed as a fissure kept opening and healing. From the Alps and Pakistan.

Made of thin, parallel fibres. Satin spar gypsum and ulexite are fibrous. Some fibrous minerals (asbestos group) should never be handled or broken.

A shallow tray of specimens sold together, usually wholesale. A "flat" is how dealers buy at shows; single pieces are picked from it.

A specimen crystallized freely in a cavity or soft medium, with no point of attachment anywhere on its surface. Complete on all sides.

Visible light emitted by a mineral while it is illuminated by ultraviolet light. Response varies by wavelength: longwave 365 nm, midwave 302 nm, shortwave 254 nm.

How a mineral breaks when it does not cleave. Quartz has a conchoidal fracture, curved like a shell.

Minerals that are soft, thin or brittle and break easily, such as acicular sprays, native silver wires or gypsum. Handle by the matrix only.

A volcanic vent that releases hot gases. The gases deposit delicate crystals such as sulphur, as at Vulcano and Vesuvius.

The minerals that come with the ore but were not mined for profit, such as quartz, calcite, fluorite or baryte. For collectors they are often the most beautiful part.

Transparent and clear enough that the crystal could, in principle, be cut as a gemstone. A significant value factor in species like tourmaline, beryl and fluorite.

A rock cavity lined internally with crystals. Unremarkable outside, crystal-lined within.

The rusty, iron-rich cap of an ore body, weathered at the surface. It points to the ore below and can hold colourful secondary minerals.

A granite altered by hot fluids, rich in quartz and mica, often with topaz, cassiterite, fluorite and wolframite. Classic in the Erzgebirge and Cornwall.

A twisted parallel growth of flat quartz crystals, like a slowly turning staircase. Smoky quartz gwindels from Alpine clefts are among the most prized quartz pieces.

The characteristic external shape a mineral tends to take: cubic, octahedral, prismatic, tabular, acicular, botryoidal, massive. A primary identification clue.

Collector shorthand for extremely fine, hair-like crystals — filiform millerite, byssolite or rutile. Spectacular under magnification, almost impossible to clean.

Heated to change or improve the colour. Most commercial citrine is heated amethyst.

A crystal with different terminations at its two ends. Hemimorphite takes its name from this feature; tourmaline shows it too.

Doubly terminated, water-clear quartz crystals from Herkimer County, New York. The name is used loosely for similar crystals from elsewhere — check the locality.

A skeletal growth where the edges of the crystal grew faster than its faces, leaving recessed, stepped, window-like hollows. Classic in halite, bismuth and some quartz.

The rock that contains a deposit or the cavity where crystals grew.

Formed from hot, mineral-rich water moving through rock. Most vein and cleft minerals — quartz, fluorite, baryte, sulfides — are hydrothermal.

A mineral whose colour comes from its own essential elements, so it is always about the same colour. Malachite (green) and azurite (blue) are idiochromatic.

The International Mineralogical Association, which approves new mineral species and their names.

Glass, resin or another material made to look like a mineral. Always buy from sellers who state what a specimen is.

Another mineral, a fluid or a gas trapped inside a crystal as it grew — rutile needles in quartz, chlorite phantoms, water bubbles (enhydro). Often adds value rather than removing it.

A rainbow-like play of colours on the surface, from a thin film or oxidation layer — iridescent hematite, chalcopyrite, "rainbow" pyrite. Fragile and easily rubbed off.

Exposed to radiation in a lab to change the colour, as with most blue topaz and some smoky quartz.

A quartz twin of two flat crystals joined at almost 85°, forming a V or heart shape. It was made famous by finds in Japan.

The card that travels with a specimen, with the species, locality and previous owners. Old labels from famous dealers or collections add value.

A flash of blue, green or golden colour across labradorite when it is turned, caused by thin internal layers.

Built of thin plates or layers stacked on top of each other, as in mica, some calcite and many feldspars.

Minerals that change or decay under light, such as realgar, proustite and vivianite, which can darken. Keep them in a dark cabinet.

The exact place where a specimen was found, as precisely as possible: mine, district, region and country. It is the most important label information.

Ultraviolet light of about 365 nm, the most common UV torch for collectors. Many fluorites, calcites and opals glow under it.

A group of specimens sold as one item, often for study, resale or for starting a collection. At ITALMINERAL, lots are tagged "Mineral Lot" in the shop.

How a surface reflects light: vitreous (glassy), adamantine (brilliant), metallic, resinous, silky, earthy. Independent of colour, and often more important to a specimen's impact.

Large, smooth, rounded bulges, bigger than botryoidal ones. Hematite and malachite often form mammillary surfaces.

A mineral occurring as a compact mass with no visible crystal faces. Judged on colour, texture and polish rather than crystal form.

ITALMINERAL's own top tier: a specimen that combines aesthetics, crystal quality, condition, rarity and locality at the highest level we handle. Few pieces earn the label.

The host rock on which crystals grew. "On matrix" indicates the specimen retains its natural geological context.

The shine of polished metal, typical of opaque minerals such as pyrite, galena and hematite.

Rocks and minerals transformed by heat and pressure without melting. Garnet, kyanite and staurolite are typical metamorphic minerals.

A small gas pocket in granite or pegmatite where crystals grew freely into open space. Source of many fine topaz, beryl and smoky quartz crystals.

A tiny specimen, under 1 cm, studied with a microscope and kept in a small box. It is the Micromount (0-1 cm) class of our Size filter.

An underground or open-pit working for ore. Many classic specimens come from mines that are now closed or flooded, which fixes the supply forever.

A naturally formed solid with a defined chemical composition and an ordered crystal structure.

A fair where dealers and collectors meet, such as Sainte-Marie-aux-Mines, Munich, Tucson, Brescia and Novegro.

A collector's term for small specimens of a few centimetres, roughly 3 to 5 cm. On our site they fall in the Thumbnail (1-5 cm) or Small Cabinet (5-8 cm) size classes.

A relative scratch-resistance scale from talc (1) to diamond (10). Practical meaning: anything below 5 scratches easily against other specimens in storage.

A specimen of the quality museums look for: exceptional size, aesthetics, condition or rarity for its locality.

Material from a recent discovery, before the market has established price levels. Prices can move sharply in either direction as the extent of the find becomes clear.

A rounded lump of mineral matter in a rock, such as flint, chalcedony or a closed geode.

Eight triangular faces, like two pyramids joined at the base. It is classic for fluorite, spinel, magnetite and diamond.

Treated with oil or resin to hide cracks or improve the shine. It must be declared.

Material mined years or decades ago and held since, often from localities no longer producing. Frequently better prepared and documented than current production.

Made of small round grains (less than 2 mm), like fish eggs. Common in some iron ores and limestones.

A milky, bluish shimmer seen in common opal and moonstone.

Rock or mineral mined because it contains a useful metal or substance, such as galena for lead.

A later layer of crystal growth on top of an earlier crystal, often of a different colour or of a different mineral.

The upper part of an ore deposit altered by air and water. It produces the colourful secondary minerals: azurite, malachite, wulfenite, mimetite and many more.

Crystals built of stacked, progressively smaller layers, like the roofs of a pagoda. Typical of some Chinese calcite with hematite.

The sequence in which minerals formed on a specimen — which grew first, which came later. Reading it explains why one crystal sits on another.

A crystal that kept its external shape while its internal crystal structure changed to a different form of the same chemical composition — for example aragonite converted to calcite.

A soft shine like the inside of a shell, usually on cleavage surfaces of apophyllite, stilbite, mica or talc.

The documented history of a specimen: previous collections, dealers and exhibitions.

A very coarse-grained igneous rock, often with pockets, that produces large crystals of tourmaline, beryl, spodumene, topaz and rare-element species.

Two crystals that grow through each other and share part of their volume. Fluorite, pyrite (iron cross), staurolite and orthoclase form penetration twins.

A small clear plastic box used to protect thumbnails and micromounts.

A ghost outline visible inside a transparent crystal, marking an interruption in growth where a thin layer of another mineral settled before growth resumed.

Light emission that continues after the UV source is switched off — from a fraction of a second to several minutes, depending on the species.

A deposit of heavy minerals concentrated by rivers or waves, such as gold, cassiterite or the gem gravels of Sri Lanka. Also called alluvial.

The shifting patches of spectral colour seen in precious opal, produced by diffraction from a regular array of silica spheres.

A crystal showing different colours when viewed along different crystallographic directions. Kunzite is a classic example, appearing pink, violet or near-colourless depending on the angle.

Feather-like aggregates of very fine crystals, as in some jamesonite, boulangerite and mica.

A natural cavity in rock where crystals grew into open space. A "pocket" in dealer language usually means a single find, and specimens from the same pocket share a family look.

A specimen or surface polished by hand or machine, such as a slice of agate. It is no longer a natural surface.

Minerals with the same chemistry but a different crystal structure. Calcite and aragonite, or diamond and graphite, are polymorphs.

Some top specimens have no public price: contact us and we will reply personally.

A mineral formed when the deposit was first created, for example galena or sphalerite in a vein.

An elongated crystal habit with parallel faces, like a pencil: tourmaline, beryl, quartz. The termination style is usually the identifying detail.

Perfect condition, with no damage at all, as it came from the ground.

The documented history of a specimen: where it was collected, who owned it, which labels travelled with it. Provenance is proof, and proof is value.

One mineral that has replaced another while preserving the original's external crystal shape. Written as "A after B" — for example hematite after magnetite.

The slow decay of some pyrite and marcasite in humid air, with white powder and a sulphur smell. Keep them dry.

A crystal with twelve pentagonal faces, typical of pyrite. Elba Island is a classic locality for it.

An open working for building stone, marble or aggregate. Quarries are accidental mineral producers — many Italian classics (Carrara, Baveno, Val Bregaglia) are quarry finds.

Crystals spreading outwards from a central point, like the spokes of a wheel. Wavellite, natrolite and pyrophyllite often radiate.

Spontaneous emission from unstable elements, principally uranium and thorium in mineral specimens. Measured in µSv/h and safely managed with sealed storage and sensible placement.

A crystal that broke in nature and then healed by growing again. It is natural, not damage.

Kidney-shaped rounded masses. The classic example is kidney ore hematite from Cumbria.

Repaired means broken pieces reattached. Restored means missing material has been added or reconstructed. Both are acceptable when disclosed; neither should ever be hidden.

On our site, a specimen already promised to a customer. You can still ask us about it on WhatsApp.

A shine like resin or honey, typical of sphalerite and sulphur.

A quartz crystal whose tip is narrower than the main body, the opposite of a normal scepter.

A crystal with six rhomb-shaped faces, like a slanted cube. Calcite, siderite and dolomite often show it.

A rose-shaped aggregate of tabular crystals. Hematite iron roses from the Alps, baryte and gypsum desert roses are classic.

Natural, uncut material, as it came from the mine.

A pointed crystal with twelve triangular faces, typical of calcite (dogtooth spar).

A crystal with a wider, larger crystal grown over the tip of a narrower stem, resembling a royal sceptre. Common in quartz and amethyst.

A mineral formed later by the alteration of primary minerals, usually in the oxidation zone. Azurite, malachite and pyromorphite are secondary.

A specimen the seller found personally in the field. Adds certainty about the locality and often a story worth keeping with the label.

A specimen that sits stably in its best display orientation without a stand. A practical, and surprisingly desirable, property in larger pieces.

A concretion cracked inside and later filled with calcite or other minerals, giving a beautiful pattern when cut.

Ultraviolet light of about 254 nm. Some minerals, like the willemite and calcite from Franklin, glow mainly under it. It needs eye protection.

A soft sheen like silk, from fine parallel fibres. Satin spar gypsum and fibrous malachite are silky.

A rock formed where hot magma reacted with limestone. A rich environment for garnet, vesuvianite, epidote, magnetite and hedenbergite crystals.

Growth where edges and corners developed faster than the faces, producing an open, hollowed or stepped framework rather than solid faces.

A specimen of about 5 to 8 cm. It is the Small Cabinet (5-8 cm) class of our Size filter.

A series of minerals in which one element gradually replaces another, such as forsterite to fayalite (olivine) or albite to anorthite (plagioclase).

A mineral with its own approved name, chemistry and structure, such as quartz or fluorite.

How heavy a mineral is compared with the same volume of water. Baryte and galena feel surprisingly heavy in the hand.

A flat, triangular twin of two octahedra, typical of spinel, magnetite and diamond (macle).

A divergent bundle of thin crystals fanning out from one point.

Treated with a consolidant so that a fragile or crumbly specimen holds together. Common for some fossils and porous minerals; always stated in our listings.

Formed as hanging, icicle-like growths — common in malachite, goethite and aragonite. A cut and polished cross-section shows concentric banding.

Star-shaped radiating groups of crystals, as in wavellite or pyrophyllite.

The colour of the powder of a mineral, seen by rubbing it on unglazed porcelain. Hematite is black or silver but leaves a red-brown streak.

Fine parallel lines on crystal faces, left by the way the crystal grew. Quartz prisms show them across, tourmaline along its length, pyrite cubes on each face.

Formed near the surface by descending water, such as the rich secondary copper minerals above many ore bodies.

A twin that looks like the tail of a swallow, classic for gypsum (selenite).

Grown in a lab with the same chemistry as the natural mineral. Colourful bismuth hopper crystals are lab-grown.

A collection organized to represent as many distinct mineral species as possible, arranged by chemical classification rather than by appearance.

Flat, plate-like crystals, wider than they are thick — baryte, wulfenite and many feldspars. Thin tabular crystals are easily chipped at the edges.

A thin coloured film on the surface of metallic minerals. It gives bornite and chalcopyrite their peacock colours and turns silver black.

A reversible colour change: some hackmanite and sodalite darken under UV and fade again in daylight. A rare and fascinating property.

The end of a crystal, closed by its natural faces. A sharp, undamaged termination is one of the first things to check.

A small specimen of about 1 to 5 cm. On italmineral.com the Size filter calls this class Thumbnail (1-5 cm).

A collection built around geography — a region, a country or a single mine — rather than around species. Provenance documentation is central to it.

A crystal with 24 four-sided faces, typical of garnet and analcime.

Any human action that changes the look of a mineral: heating, irradiation, dyeing, oiling, coating. Honest sellers always declare it.

Flashes of light produced when a crystal is struck, rubbed or crushed — seen in some sphalerite, fluorite and quartz. Not something to test on a good specimen.

Excess matrix removed with a saw or trimmer to improve proportions. Standard practice, and the cut surfaces are normally kept out of the display view.

A small stone rounded and polished in a rotating barrel.

Two or more crystals of the same species intergrown in a specific, symmetrical orientation defined by the crystal structure — not a random intersection.

The place where a mineral species was first discovered and formally described. Specimens from a type locality carry particular scientific and collector significance.

Replacing a specimen in a collection with a better one of the same species or locality.

A named type of a species with a special colour or habit. Amethyst, citrine and smoky quartz are varieties of quartz.

A crack in rock filled with minerals deposited by hot fluids. Many classic specimens come from veins.

The glassy shine of most transparent minerals, such as quartz, beryl and fluorite.

Minerals that dissolve in water, such as halite, sylvite and chalcanthite. Never wash them.

Tourmaline with a pink or red core and a green rim, looking like a slice of watermelon when cut across.

Buying in quantity at a reduced per-piece price, typically by flat or lot. ITALMINERAL supplies dealers and resellers on request.