Minerals, their chemistry and where they form
Browse the minerals we collect, cut and study — what they are made of, how hard they are, and the geological processes that created them.
31 entries
Amethyst's colour fades in strong sunlight and changes to golden when heated, which is how most commercial citrine is made.
Geological Origin
A purple variety of quartz coloured by trace iron. It grows inside geodes and vugs in volcanic lava flows where silica-rich water deposits crystals at low temperature, and in hydrothermal veins.
Brazil, Uruguay, Zambia; Namibia
Also called trystine or bolivianite. True natural ametrine comes from a single Bolivian mine.
Geological Origin
A naturally zoned mixture of amethyst and citrine, formed where a temperature gradient across a growing crystal changed the oxidation state of its iron as it grew.
Anahí Mine, Bolivia
Named after Anglesey in Wales, where it was first described.
Geological Origin
A secondary lead mineral formed where galena oxidises in the presence of sulphate-bearing groundwater, common in the upper zones of lead deposits.
Tsumeb (Namibia), Sardinia, Scotland
Frequently found with malachite and slowly alters into it, which is why many specimens show green fringes.
Geological Origin
A secondary copper mineral formed in the oxidised zone above copper sulphide ore bodies, where descending groundwater reacts with chalcopyrite and bornite.
Tsumeb (Namibia), Morocco, Arizona
Emerald is the green gem variety, aquamarine the blue. Beryl is the principal ore of beryllium.
Geological Origin
Forms in beryllium-bearing pegmatites and in schists and limestones altered by hydrothermal fluids. Emerald's vivid green comes from chromium or vanadium substituting into the structure.
Kagem, Zambia; Colombia; Brazil
Prized by lapidaries for its delicate, lace-like banding and its soft blue colour.
Geological Origin
A banded chalcedony formed where silica-rich groundwater filled cavities in volcanic rock, the fine layers recording successive episodes of deposition.
Ysterputs (Namibia), South Africa
First described from the Bultfontein mine in the Northern Cape, after which it is named.
Geological Origin
A rare calcium silicate that forms where hydrothermal fluids alter calcium-rich rock in contact with manganese ore.
Kalahari Manganese Field (Northern Cape)
Defines hardness 3 on the Mohs scale and shows perfect rhombohedral cleavage in three directions.
Geological Origin
Forms the bulk of sedimentary limestone and metamorphic marble, and precipitates from calcium-rich groundwater in caves, hot springs and hydrothermal veins.
Kalahari Manganese Field, Tsumeb, worldwide
Its very high specific gravity makes it surprisingly heavy in the hand for a pale mineral.
Geological Origin
A secondary lead mineral crystallising in the oxidised zone above galena-bearing veins, where carbonate-rich water reacts with the primary ore.
Tsumeb (Namibia), Australia, Sardinia
Named from the French citron, after its lemon-yellow colour.
Geological Origin
Yellow quartz coloured by trace iron, forming in hydrothermal veins. Most citrine sold today is heat-treated amethyst, where heating changes the oxidation state of the iron from purple to gold.
Brazil, Madagascar, Spain
A major source of borax. It is fluorescent in short-wave UV, showing pale blue or white.
Geological Origin
An evaporite mineral precipitated in boron-rich, alkaline lake basins, and later in hydrothermal veins.
California, Turkey, Argentina
Often coated by a thin film of malachite; the deep red internal reflections are prized in specimens.
Geological Origin
A secondary copper oxide formed in the oxidised zone of copper deposits, crystallising where cuprous solutions are present with little available sulphur.
Onganja (Namibia), Tsumeb, Cornwall
Strongly fluorescent in short-wave ultraviolet light, and hard enough to be faceted as a gem.
Geological Origin
A boron-bearing silicate crystallising in hydrothermal veins and in metamorphosed limestones and evaporites.
Mexico, Myanmar, Japan
Many specimens glow under ultraviolet light — the word fluorescence takes its name from this mineral.
Geological Origin
Precipitates from hydrothermal fluids in veins and cavities, especially around lead-zinc deposits and in granite. It is also a common gangue mineral in Mississippi Valley-type ore fields.
Riemvasmaak (Northern Cape), Namibia, England
Crystallises in cubes with perfect cleavage, and tarnishes to a dull grey on exposure.
Geological Origin
The principal ore of lead, deposited from hot hydrothermal fluids in veins and in sedimentary carbonate rocks. It commonly carries silver as an impurity.
Tsumeb, Missouri, Broken Hill
The main ore of iron. Its earthy red streak has been used as a pigment since prehistory.
Geological Origin
Precipitates from iron-rich water in sedimentary banded iron formations, and forms by hydrothermal alteration and weathering of other iron minerals.
Sishen (Northern Cape), Brazil, Australia
Because it is tough and takes a good polish, jasper has been used for tools and ornaments for tens of thousands of years.
Geological Origin
An opaque, microcrystalline form of quartz coloured by iron oxides and other impurities. It precipitates from silica-rich water in veins, and occurs in sedimentary and volcanic rocks.
Worldwide; common in South African banded iron formations
Described as recently as 1965 and prized by collectors for its bright, sharp microcrystals.
Geological Origin
A rare hydrated calcium-manganese carbonate-sulphate that crystallises late in the oxidised, manganese-rich zones of deposits such as the Kalahari Manganese Field.
Kalahari Manganese Field (Northern Cape)
An important ore of lithium. Its lilac colour comes from traces of manganese.
Geological Origin
A lithium-bearing mica that crystallises in lithium-rich pegmatites late in their cooling history.
Brazil, Zimbabwe, Madagascar
One of the earliest copper ores worked by humans, and still a popular lapidary and decorative stone.
Geological Origin
Another secondary copper mineral of the oxidised zone, forming as groundwater alters copper sulphides. Its concentric banding records successive layers of growth.
DR Congo, Tsumeb, Russia
One of the feldspars, the most abundant mineral group in the Earth's crust.
Geological Origin
A primary mineral of granite and other felsic igneous rocks, crystallising directly from cooling magma. It also grows in pegmatites and in some metamorphic rocks.
Madagascar, Sri Lanka; worldwide granite terrains
Known as fool's gold for its misleading golden colour. Striking it against steel throws sparks.
Geological Origin
Forms in hydrothermal veins, in sedimentary and metamorphic rocks, and as framboids in anoxic mud. It is the most common sulphide mineral on Earth.
Worldwide
The most abundant mineral in the crust after feldspar. Hard, chemically resistant and piezoelectric, which is why it appears in everything from gemstones to watch movements.
Geological Origin
Crystallises from silica-rich hydrothermal fluids and melts, most commonly in veins, pegmatites and vugs where hot, mineral-laden water cools. It also forms as a primary mineral in granite and survives weathering to become the main component of most river and beach sand.
Worldwide; abundant in the Cape Fold Belt and Namibian pegmatites
Usually found massive rather than as well-formed crystals, since it is typically formed under stress.
Geological Origin
Pink quartz coloured by traces of titanium, iron or manganese. It forms in pegmatites and hydrothermal veins, and massive rose quartz can occur in very large bodies.
Northern Cape pegmatites, Brazil, Madagascar
So light and porous when dry that it floats on water; long carved into pipe bowls under the name meerschaum.
Geological Origin
A fibrous clay mineral formed by the alteration of magnesium-rich rocks such as serpentinite, and precipitated in lake beds.
Eskişehir (Turkey), Spain, Kenya
Common in granite pegmatites and Alpine-type veins, and often associated with radioactive minerals.
Geological Origin
Quartz darkened by natural irradiation of aluminium impurities in the crystal lattice.
Switzerland, Brazil, Scotland
The main ore of zinc. Its pale varieties have a diamond-like lustre and very high dispersion.
Geological Origin
A primary zinc sulphide that precipitates from hydrothermal fluids, typically alongside galena in veins and in carbonate-hosted deposits.
Aliva (Spain), Tsumeb, Trepča
Also called cactus quartz. The classic material comes from the Boekenhout area of South Africa.
Geological Origin
A variety of amethyst in which a second generation of quartz grew over the first, splaying outwards in a cactus-like habit.
Boekenhout (Gauteng), South Africa
Develops in banded iron formations under metamorphism; South Africa's Griqualand West deposits are world-famous.
Geological Origin
Formed when quartz replaces fibrous crocidolite (blue asbestos) while preserving the fibre direction, producing a shifting, silky chatoyancy.
Northern Cape (Griqualand West), Australia
One of the few minerals that is strongly pyroelectric and piezoelectric — heating or squeezing a crystal produces an electric charge.
Geological Origin
A boron-bearing silicate that crystallises from volatile-rich granitic pegmatites and hydrothermal veins, and in metamorphic rocks such as schist and marble.
Namibia, Madagascar, Brazil, Mozambique
Noted for its hexagonal, barrel-shaped crystals and its striking red-orange colour.
Geological Origin
A secondary lead vanadate crystallising in the oxidised zones of lead deposits where vanadium-bearing solutions are present.
Mibladen (Morocco), Arizona, Namibia
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