Achroite
Tourmaline
Achroite is the colourless variety of elbaite, the lithium tourmaline species behind most gem tourmaline. It is defined by an abse…
The stones
Every stone, one entry at a time
A reference entry for each gem material we can document properly: chemical formula, crystal system, hardness, refractive index, specific gravity, what causes the colour, and where it is actually found. Where a figure is not established, we say so rather than inventing one.
A15
Agate
Quartz (chalcedony)
Agate is the banded variety of chalcedony, the microcrystalline form of quartz, formed by successive layers of silica deposited in…
Alexandrite
Chrysoberyl
Alexandrite is the colour-change variety of chrysoberyl: green to bluish-green in daylight, red to purplish-red under incandescent…
Almandine
Garnet group
Almandine is the iron aluminium member of the garnet group and the species behind most dark red garnet ever cut. It is untreated,…
Amazonite
Feldspar
Amazonite is the green to blue-green variety of microcline, a potassium feldspar. Its colour comes from trace lead and structural…
Amber
Organic gem
Amber is fossilised tree resin, hardened and polymerised over millions of years. It is organic, amorphous, and at a specific gravi…
Amethyst
Quartz
Amethyst is the purple variety of quartz, coloured by iron in combination with natural irradiation. It is the most abundant of all…
Ametrine
Quartz
Ametrine is a single quartz crystal that is amethyst in one part and citrine in the other, both colours produced by iron in differ…
Ammolite
Organic gems
Ammolite is the iridescent fossilised nacre of Upper Cretaceous ammonites, cut as an organic gem. Its colour is structural, produc…
Andalusite
Andalusite
Andalusite is an aluminium silicate, Al₂SiO₅, cut as a gem for its exceptionally strong pleochroism, which shows green from one di…
Andradite
Garnet
Andradite is the calcium-iron garnet, Ca₃Fe₂(SiO₄)₃, a cubic mineral whose dispersion of 0.057 is higher than diamond's. Its varie…
Apatite
Apatite
Apatite is a calcium phosphate, Ca₅(PO₄)₃(F,Cl,OH), and the mineral that defines point 5 on the Mohs scale. It is cut for colours…
Aquamarine
Beryl
Aquamarine is the blue to blue-green variety of beryl, the same species as emerald, coloured by iron. Unlike emerald it grows clea…
Aventurine
Quartz
Aventurine is a translucent massive quartz, coloured and made to glitter by platy mineral inclusions: chromium-bearing fuchsite mi…
Azurite
Carbonates
Azurite is a copper carbonate hydroxide, Cu₃(CO₃)₂(OH)₂, and the deep blue partner of malachite in the oxidised zone of copper dep…
B6
Baltic Amber
Fossil resins
Baltic amber is fossilised tree resin from the Eocene forests of northern Europe, recovered chiefly from the glauconitic sands kno…
Benitoite
Silicates
Benitoite is a rare barium titanium silicate, found as transparent blue crystals in commercial quantity at a single mine in San Be…
Bi-colour Tourmaline
Silicates — tourmaline group
Bi-colour tourmaline is a single elbaite crystal carrying two or more distinct colours as growth zones, most often green and pink.…
Black Opal
Silica
Black opal is precious opal with a dark body tone, graded N1 to N4 on the Australian scale, against which its play-of-colour reads…
Bloodstone
Silica
Bloodstone is an opaque dark green chalcedony carrying discrete red spots of iron oxide, known since antiquity as heliotrope. It i…
Blue Topaz
Topaz
Blue topaz is topaz — aluminium fluorosilicate, hardness 8 — in any blue tone. Natural blue topaz is genuine but scarce and always…
P2
Padparadscha
Corundum
Padparadscha is a variety of corundum — the same mineral as ruby and sapphire — defined by a delicate mixture of pink and orange a…
Paraíba Tourmaline
Tourmaline
Paraíba tourmaline is elbaite coloured by copper and manganese, in a defined range from electric blue through blue-green to green.…
T2
Tourmaline
Tourmaline group
Tourmaline is a group of more than thirty borosilicate mineral species that together produce a wider colour range than any other g…
Tsavorite
Garnet group
Tsavorite is the green, vanadium- and chromium-coloured variety of grossular garnet, found in a narrow geological belt running thr…