Smoky quartz

Smoky quartz is quartz coloured grey to brown to nearly black, and the mechanism is genuinely different from the other varieties. Aluminium substitutes for a small fraction of the silicon in the lattice; natural radiation from the surrounding rock — potassium, uranium and thorium over geological time — knocks an electron loose at those sites, and the resulting colour centre absorbs light.

So the colour is a record of radiation dose and of time. Heating it above about 300 °C returns the electrons and the colour disappears; irradiating colourless quartz containing aluminium produces it artificially, and a great deal of very dark commercial material has been made that way.

It is the state gem of New Hampshire and, as cairngorm, the traditional stone of Scottish jewellery and dirk handles.

Brown to black transparent quartz with the species' hardness and conchoidal fracture. Distinguished from smoky topaz — which is a misnomer for this material — and from andradite garnet by hardness and density.

Irradiated material is often very dark, evenly coloured and slightly greyer; natural stones tend to be paler and to vary through the crystal.

Gemstone, carvings, and ornamental use. Historically, thin polished slices were used as sunglasses in China from the twelfth century, which is one of the earlier recorded uses of a material for eye protection.

Descriptive in English. The Scottish name cairngorm is from the Cairngorm mountains, where it was collected, and it is the older name in British use.