Quartz, silica, or silicon dioxide,
Formula SiO₂.

Crystal system: Hexagonal.
Hardness: 7 gms/cm³
Density: 2.7
Colour: Colourless or white (inclusions may cause many colours, red, pink, yellow, green, mauve and brownish-black).
Streak: Reddish brown.

Quartz is the most widespread and abundant mineral in the Earth's crust. It occurs in macrocrystalline, microcrystalline (chalcedony) and hydrated (opal) forms. The more common macrocrystalline forms are rock crystal (clear), citrine (yellow), amethyst (mauve), rose quartz (pink) and smokey quartz (brown). Quartz has many uses; as sandstone in construction; for glass making; in the electronics industry as extremely accurate crystal oscillators for computers and watches.

The picture shows a nice specimen of quartz tinted pink by the inclusion of iron, with which quartz is often associated. The specimen is from Guangdong in China.