Mineral varieties
Named varieties of mineral species, distinguished by colour or habit rather than by composition. Each one links to the species it belongs to.
Agate
Banded chalcedony — quartz in crystals too small to see, deposited in rhythm inside a cavity.
Amazonite
Green microcline, coloured by lead — and it is named after a river it does not come from.
Aquamarine
Beryl coloured blue by iron — the same mineral as emerald, grown somewhere calmer, and usually clean because of it.
Carnelian
Orange-red chalcedony coloured by iron oxide — the stone of Roman signet rings, because hot wax does not stick to it.
Citrine
Yellow quartz — and most of what is sold as it is heated amethyst, which the trade discloses and shoppers rarely hear.
Emerald
Beryl coloured green by chromium — and the one major gemstone whose inclusions are expected rather than a fault.
Jasper
Opaque chalcedony full of other minerals — and it is the impurities that make it worth looking at.
Kunzite
Pink spodumene — a difficult stone to cut, and one whose colour can fade in daylight.
Moonstone
Feldspar with a blue-white glow that floats under the surface — and it is two intergrown feldspars scattering light between them.
Morganite
Pink beryl, coloured by manganese — named for a banker, and a gemstone that became fashionable a century after it was described.
Peridot
Gem olivine, and one of very few gemstones that occurs in only one colour — including specimens that arrived from space.
Rose quartz
Pink quartz, and the colour is microscopic fibres of another mineral inside it rather than an impurity in the quartz.
Smoky quartz
Quartz turned brown by natural radiation acting on traces of aluminium — the only common gemstone whose colour is a radiation record.
Tiger's eye
Quartz that has replaced asbestos fibre and kept its shape — which is where the silky moving band of light comes from.