Quartz is one of the most common minerals. It consists of silicon dioxide, or silica. It is found in many places and in many types of rock formations. Several varieties of quartz are used as gems. Other s are building materials. Quartz is very hard.
   There are many colors and many varieties of quartz. The color is due to other minerals and determines certain varieties. Rock crystal is pure, clear, transparent quartz. Pur­ple quartz is called amethyst. These are crystalline varieties. Agate is an opaque type in which the color is distributed unevenly through its mass. Sometimes the color runs through in curved bands. In moss agate specimens, the color is arranged so that it looks as if moss or other vegetable matter has been "frozen" in the quartz. Agate, carnelian, and onyx are usually considered as special types of quartz—chalcedony.
   Chalcedony is the massive form of quartz. Other varieties include bloodstone, flint, jasper, sand and gravel, sandstone, and quartzite.