Quartz

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 was a common gemstone even in Pliny's time. Never the less he was concerned enough about the introduction of glass to complain about it possibly depressing the price of good crystal.

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.

Quartz Families

Most of the quartz varieties listed above are transparent. There are also several translucent or opaque varieties that make good gemstones. Most of these were unknown in medieval times, at least as separate minerals. They were often classed with other gemstones of similar appearance.

• aventurine (a pale to vivid green variety of quartz, heavily included with mica; it was unknown in medieval times but was most likely classed as green jasper; the Chinese sometimes used it as a substitute for jade)
• blue quartz (unlike blue crystal above, blue quartz is a natural stone but rather turbid; it takes its color from inclusions of crocidolite or rutile; the color ranges from gray to sky blue and gem quality material is hard to obtain)
• dumortierite quartz (medium to dark blue in color; it is often mistaken for lapis lazuli)
• tiger's-eye (golden brown, inclusions of asbestos needles give it a chatoyancy almost like that of a holograph; apparently unknown in medieval times, it may have been the source of Pliny's "hyena stone")