Smoky Quartz

Smoky quartz forms in many geological environments but is most characteristic of pockets in igneous rocks and pegmatites; the quartz grains in granites are often smoky. In Scotland, radioactive uranium and thorium concentrated in the Cairngorm granite turned quartz crystals smoky, forming the brown "Cairngorm" quartz. Gemdat lists significant gem localities in Bolivia, Brazil, Kenya, Madagascar, Mozambique and Myanmar, among others.

Smoky quartz has the gemological constants of quartz (refractive index 1.544–1.553, birefringence 0.009, specific gravity 2.65). True smoky quartz is dichroic in polarized light, which separates it from black or brown quartz coloured by inclusions of minerals or organic matter; such material is often wrongly called smoky quartz or morion. Colour produced by irradiating colourless quartz is not detectable with standard tests.

The colour of smoky quartz is sensitive to heat: it pales at temperatures above 200–300 °C and with prolonged exposure to UV light. Keep it away from high heat and long exposure to strong sunlight. Like other quartz it has no cleavage but is brittle.

According to National Museums Scotland, smoky quartz from the Cairngorm mountains in Scotland became known as "cairngorms", a name documented from at least 1780 in lapidaries' newspaper adverts and in letters of Sir James Grant, Chief of Clan Grant. From the late 18th century cairngorms were used in seals, buttons and brooches and decorated weapons and accessories of Highland dress; the museum describes them as conveying ideas of ancestry and ties to the landscape.