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Common, coarse-grained variety of 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.
This entry comes from Encyclopædia Britannica Concise. For the full entry on smoky quartz, visit Britannica.com.