The second most common mineral in the Earth's crust after feldspar, it is made up of a lattice of silica tetrahedra. Quartz has a hardness of 7, which makes it attractive to the jeweller as a robust gem.
Quartz belongs to the rhombohedral crystal class and the normal crystal is a six-sided prism terminating with six-sided pyramids at each end. In nature quartz crystals are often twinned, distorted, or so intergrown with neighbouring crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal facets altogether and appear as large masses. Well-formed crystals typically form in a 'bed' or in hollows called geodes.
Pure quartz is colourless or white. Coloured varieties include rose quartz, amethyst, smoky quartz and others. Colours apart, the most important distinction between types of quartz is based upon the size of the individual crystals. Macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (individual crystals visible only under high magnification). The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline.
Chalcedony - Any cryptocrystalline quartz, although generally only used for white or lightly coloured material. Otherwise more specific names are used.
Agate - Multi-coloured, banded chalcedony, semi-translucent to translucent.
Onyx - Agate where the bands are straight, parallel and consistent in size.
Jasper - Opaque chalcedony, typically red to brown.
Aventurine - Translucent chalcedony with small inclusions (usually mica) that shimmer.
Tiger's eye - Fibrous gold to red-brown coloured quartz, exhibiting chatoyancy.
Rock crystal - Clear, colourless
Amethyst - Purple, transparent.
Citrine - Yellow to reddish orange to brown, greenish yellow.
Prasiolite - Mint green, transparent.
Rose quartz - Pink, translucent, may display diasterism.
Rutilated quartz - Contains acicular (needles) inclusions of rutile.
Milk quartz - White, translucent to opaque, may display diasterism.
Smoky quartz - Brown to grey, opaque.
Morion - Dark-brown, opaque.
Carnelian - Reddish orange chalcedony, translucent