QUARTZ

The fundamental structural unit of industrial silicate minerals is the silica tetrahedron. Quartz is just a densely packed arrangement of these tetrahedra, as depicted in Figure 3. 

Quartz

Extended in three dimensions, this structure provides the characteristic hardness and inertness of quartz. The different forms of crystalline silica − most commonly quartz, cristobalite, and tridymite − differ mainly in the relative orientation of adjacent tetrahedra and the shape of voids created within a given plane.