A very common, coarse-grained variety of the silica mineral quartz that ranges in colour from nearly black through smoky brown. No distinct boundary exists between smoky and colourless quartz. Its abundance causes it to be worth considerably less than either amethyst or citrine. Heating bleaches the stone, the colour sometimes passing through yellow; these yellow pieces are often sold as citrine. Crystals of the mineral frequently contain inclusions of gas (carbon dioxide), liquid (often both water and liquid carbon dioxide), or solids (rutile). Smoky quartz from Mount Cairngorm, Scotland, is known as cairngorm and is a favourite ornamental stone in
Scotland, where it is worn in brooches with Highland costume. Its properties are those of other quartz stones. Today, the vast majority of smoky quartz is mined in Brazil, but it is found in most of the major gem producing nations.
Smoky quartz is quite often sold in the trade as smoky topaz. 99% of the time a gem offered as smoky topaz is actually this variety of quartz. Since topaz is a separate mineral, this type of name can be confusing and should be clearly disclosed as to which gem is actually being presented, since topaz also occurs in a brown color.