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. Even so, he noted that that a rock crystal bowl sold for 150,000 sestertii (about $6000 US). He also expressed some surprise that the introduction of glass did not adversely affect the price of crystal. Today, fine glassware is still referred to as "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.
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 probably available and 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")
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 (SiO2•nH2O)
Quartz is a fairly hard mineral, ranking 7 on the Mohs scale. This is an important property for reenactors to note. Because silica is commonly found in ordinary dust, any gemstone softer than quartz will eventually lose its polish. Extra care should be given to softer stones, such lapis lazuli, in order to prevent scratching. In addition, quartz is very durable, having no cleavage planes. This makes it suitable for all types of jewellry. Microcrystalline quartz (agate, chalcedony, etc) is somewhat softer (6½ to 7 on the Mohs scale), but is still very durable and will hold a polish for a long time.
- C. Plinius Secundus, 37th Book of The Natural History of the World, trans. by Philomen Holland, 1601.
- Walter Schumann, Gemstones of the World, New York, 1997
- June Culp Zeitner, Gem & Lapidary Materials, Geoscience Press, Inc. 1996