Quartz

  

Quartz is the second most abundant mineral in Earth's crust after feldspar. It occurs in nearly all acid igneous, metamorphic, and sedimentary rocks. It is an essential mineral in such silica-rich felsic rocks as granites, granodiorites, and rhyolites.

• Varieties:Quartz varieties are rock crystal, milk quartz, amethyst, citrine, ametrine, smoky quartz, morion, tiger's eye, hawk's eye, aventurine. Microcrystalline quartz varieties are jasper, carnelian, chalcedony, onyx, agate, chert, flint. Structural varieties are tridymite, cristobalite, coesite, and stishovite.
• Sources: China, Japan, and Russia are the world's primary producers of quartz. Belgium, Brazil, Bulgaria, France, Germany, South Africa, and the United Kingdom also mine significant quantities of the mineral.
• Toughness: This mineral has a hardness of 7 to 7.5 on the Mohs scale.
• Colours:: citrine (yellow); amethyst (purple); rock crystal (colourless); rose quartz (pink); and smokey quartz (purplish-brown).
• Cuts & Uses: The angle to the main crystal axes at which a crystal blank is cut has a major impact on its performance: frequency stability, activity, Q, temperature coefficient, etc.
• History: Quartz has attracted attention from the earliest times; water-clear crystals were known to the ancient Greeks as krystallos—hence the name crystal, or more commonly rock crystal, applied to this variety. The name quartz is an old German word of uncertain origin first used by Georgius Agricola in 1530.