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Quartz is a crystalline form of silica (SiO2) and is one of the most common gemstones in the world. The name itself comes from the Old English word for "right-angle stone". However, some authorities believe it derives from the Slavic word for "hard". The ancient Europeans believed quartz to be a petrified form of water; hence the name crystalus, from the Greek word for "ice".

Quartz comprises an entire family of gemstones, which are classified according to color:

  • rock crystal (clear quartz)
  • smoky quartz (also known as morion & caingorm; formerly sold under the trade name "smoky topaz")
  • amethyst (purple quartz; also amethyst quartz, which is purple with translucent bands of white quartz)
  • citrine (golden yellow to scarlet red quartz; when occuring in the same stone as amethyst, it is known as ametrine)
  • rose quartz (translucent quartz that is pale to rose pink in color)

There are also man-made varieties of quartz, such as prasiolite, which is pale green, and blue crystal, a vivid cobalt blue in color.