To describe a new mineral, the structure and composition must be described. Most of these 5000 minerals are rare, so that only about 200 are common enough to make up macroscopic rocks. A crystal is a three-dimensional periodic array of atoms. Most solids are crystalline, but some, like glass, opal, amber, wood, and coal, are not and are said to be amorphous . The basic repeat unit of a crystal or mineral is the unit cell .
Crystals have complex symmetries.
A given compound like SiO 2 may occur as different crystalline forms. If the crystalline structures of the two forms are different, they are said to be polymorphs. SiO 2 may occur as the mineral quartz, or, at high temperatures quartz transforms to tridymite, and then to cristobalite. If a rock containing quartz is subducted into the mantle, the quartz transforms first to coesite at a depth of about 100km, then to stishovite at a depth of about 250km. These minerals are said to be polymorphs of quartz.