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White-tipped amethyst from Guerrero, Mexico—Lee Boltin
Transparent, coarse-grained variety of quartz that is valued as a semiprecious gem for its violet colour. It contains a little more iron oxide (FeO) than any other variety of quartz, and its colour probably arises from this iron content. Heating removes the colour or changes it to the yellow of citrine; most commercial citrine is made in this manner. Notable deposits are found in Brazil, Uruguay, Ontario, and North Carolina. The birthstone for February, amethyst is usually faceted with step cuts or emerald cuts but also has been used since ancient times for carved intaglios.
This entry comes from Encyclopædia Britannica Concise. For the full entry on amethyst, visit Britannica.com.