Gemstone
A gemstone is a mineral that has been judged worth cutting, and the judgement is cultural before it is physical. Hardness matters because a stone worn daily has to survive it, and rarity matters because scarcity is most of the price — but neither decides the question. Amber is fossil resin, pearl is made by an animal, and jet is coal, and all three have been treasured for millennia.
The one worth knowing is species against variety. Ruby and sapphire are both corundum, differing by which trace element coloured them; amethyst and citrine are both quartz. A jeweller sells varieties and a mineralogist counts species, and most confusion about gemstones starts there.
Mineral variety
Agate
Banded chalcedony — quartz in crystals too small to see, deposited in rhythm inside a cavity.
Mineral variety
Amazonite
Green microcline, coloured by lead — and it is named after a river it does not come from.
Mineral variety
Amethyst
The purple variety of quartz, coloured by iron impurities and natural irradiation.
Mineral variety
Aquamarine
Beryl coloured blue by iron — the same mineral as emerald, grown somewhere calmer, and usually clean because of it.
Mineral variety
Carnelian
Orange-red chalcedony coloured by iron oxide — the stone of Roman signet rings, because hot wax does not stick to it.
Mineral variety
Citrine
Yellow quartz — and most of what is sold as it is heated amethyst, which the trade discloses and shoppers rarely hear.
Mineral
Diamond
Carbon in a cubic lattice — the hardest naturally occurring substance, and one of the best thermal conductors known.
Mineral variety
Emerald
Beryl coloured green by chromium — and the one major gemstone whose inclusions are expected rather than a fault.
Material
Ivory
historically, and the role is recorded as history rather than as a recommendation — international commercial trade in elephant ivory has been banned since 1990
Mineral variety
Jasper
Opaque chalcedony full of other minerals — and it is the impurities that make it worth looking at.
Material
Jet
carved and faceted rather than cut in the usual sense, and the only major gem material soft enough to be finished with a knife
Mineral variety
Kunzite
Pink spodumene — a difficult stone to cut, and one whose colour can fade in daylight.
Mineral variety
Moonstone
Feldspar with a blue-white glow that floats under the surface — and it is two intergrown feldspars scattering light between them.
Mineral variety
Morganite
Pink beryl, coloured by manganese — named for a banker, and a gemstone that became fashionable a century after it was described.
Rock
Obsidian
cut and polished as a black ornamental stone, and used for mirrors long before glass ones existed
Material
Pearl
and the only one that arrives finished — a pearl is not cut, because cutting it would remove the surface that is the whole of its value
Mineral variety
Peridot
Gem olivine, and one of very few gemstones that occurs in only one colour — including specimens that arrived from space.
Mineral variety
Rose quartz
Pink quartz, and the colour is microscopic fibres of another mineral inside it rather than an impurity in the quartz.
Mineral variety
Sapphire
Corundum in any colour other than red — most famously blue, from iron and titanium.
Mineral variety
Smoky quartz
Quartz turned brown by natural radiation acting on traces of aluminium — the only common gemstone whose colour is a radiation record.
Mineral variety
Tiger's eye
Quartz that has replaced asbestos fibre and kept its shape — which is where the silky moving band of light comes from.
Mineral
Topaz
hard enough to wear well, though its one perfect cleavage makes it vulnerable to a knock
Each of these uses is established and cited on its own. Whether something counts as an ore, for instance, depends on grade, price and era — and those qualifications are shown alongside it.