Quartz is a complex silicate in which all the oxygen atoms of the SiO4 tetrahedra are shared between two tetrahedra, leading to complex 3-dimensional frameworks. For this reason, quartz is referred to as a framework silicate.

Quartz is among the most common of all rock forming minerals and is found in many metamorphic rocks, sedimentary rocks, and those igneous rocks that are high in silica content such as granites and rhyolites. It is a common vein mineral and is often associated with mineral deposits.

Cryptocrystalline varieties are used as semi-precious stones and for ornamental purposes. These varieties are divided more by character than by colour. Chalcedony, or agate, is divided into innumerable types that have been named for locally common varieties. Some of the more beautiful types have retained their names while other names have faded into obscurity. Some of the more common are chrysoprase (a pure green agate), sard (a yellow to brown agate), sardonyx (banded sard), onyx (black and white agate), carnelian (a yellow to orange agate), flint (a colourful and microscopically fibrous form), jasper (a colourful impure agate) and bloodstone (a green with red speckled agate).

quartz (single crystal)

Chemical composition - SiO2
Hardness - 7
Specific gravity - 2.65
Transparency - Transparent to translucent
Colour - Clear is most common (pure quartz), also white or cloudy (milky quartz); but can be very variable - purple (amethyst), pink (rose quartz), grey or brown to black (smoky quartz) are also common; yellow to orange (citrine) are more rare; cryptocrystalline varieties can be multicoloured
Streak - White
Lustre - Vitreous
Cleavage/fracture - Non-existent / conchoidal
Crystal habit/mode of occurrence - Prismatic (hexagonal prism terminated with a six sided pyramid) / cryptocrystalline, massive

Other specimens - Click the thumbnails to enlarge

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