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'''Quartz''' is a crystalline form of ''silica'' (SiO<sub>2</sub>) 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 silken luster and a ''chatoyancy'' almost like that of a holograph; apparently unknown in medieval times, it may have been the source of [[Pliny]]'s "hyena stone")

== Microcrystalline Quartz ==
There are several families of quartz, whose crystal structure is too small to be seen with the naked eye. These types all have a waxy luster and are translucent rather than transparent.
*[[agate]] (characterised by alternating wavy bands of color; onyx is a type of ''agate'')
*chalcedony (a variety of agate displaying solid uniform color)
*jasper (in ancient times a catch-all term for gemstones otherwise unclassified; by medieval times the term was more or less restricted to spotted green stones; today ''jasper'' refers to a variety of microcrystalline quartz that is opaque rather than translucent, and displaying more earth-like colors than either ''agate'' or ''chalcedony'')
*[[opal]] (technically opal is neither a variety of quartz, nor is it even a crystal; rather it is a silicate suspension in a hydrous gel (SiO<sub>2</sub>•nH<sub>2</sub>O)
[[category:gemstones]]