a name often given by jewellers to brown or yellow quartz or rock-crystal, because found among the Cairngorm Mountains in Aberdeenshire. [Celt. carn.]
Chambers's Twentieth Century Dictionary (1908)
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Smoky quartz: a grey, translucent variety of quartz that ranges in clarity from almost complete transparency to an almost-opaque brownish-gray or black crystal. Like other quartz gems, it is a silicon dioxide crystal. The smoky colour results from free silicon formed from the silicon dioxide by natural irradiation.
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Smoky quartz is common and was not historically important, but in recent times it has become a popular gemstone, especially for jewellery.