Quartz is the second most abundant mineral in the Earth's continental crust (after feldspar). It is made up of a framework of silicon-oxygen tetrahedra SiO4, with each silicon shared between two oxygens to give the overall formula SiO2. Quartz has a hardness of 7 on the Mohs scale and a density of 2.65 g/cm³.

Major varieties

Although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. This does not always hold true.

Major Varieties of Quartz

Chalcedony Cryptocrystalline quartz and moganite mixture. The term is generally only used for white or lightly colored material. Otherwise more specific names are used.

Agate Multi-colored, banded chalcedony, semi-translucent to translucent

Onyx Agate where the bands are straight, parallel and consistent in size.

Jasper Opaque cryptocrystalline quartz, 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, colorless

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

Carnelian Reddish orange chalcedony, translucent


Occurrence,
Quartz is an essential constituent of granite and other felsic igneous rocks. It is very common in sedimentary rocks such as sandstone and shale and is also present in variable amounts as an accessory mineral in most carbonate rocks. It is also a common constituent of schist, gneiss, quartzite and other metamorphic rocks. Because of its resistance to weathering it is very common in stream sediments and in residual soils.

Quartz occurs in hydrothermal veins as gangue along with ore minerals. Large crystals of quartz are found inpegmatites. Well-formed crystals may reach several meters in length and weigh hundreds of kilograms.

from Wikipedia