Silicon occurs in Nature combined with oxygen, in the form of silicon dioxide, and with oxygen and several metals, in the silicate form, but is never found isolated. As a whole, the silicates and the silica represent 60% of the terrestrial crust. The SiO4 is the primary structural unit of all these substances. Actually, the silicates are a very extensive group of compounds, resulting from the combination of complex metallic ions or complex negative ions with SiO4 units. There are mineral silicates of almost all the 42 metals and non-metals besides the rare-earths.
The silicon dioxide, commonly called silica, is one of the most important silicon compounds occurring in Nature. This compound occurs in three different crystalline forms: quartz, tridymite and cristobalite. These last two can only be found in volcanic rocks and do not have industrial applications. During the test of the first atomic bomb in New Mexico other silicon crystalline form appeared: the keatite. The quartz is very common and it can be found in granite, sand and sandstone. It is a piezoelectric substance used to stabilize amplifier circuits, to measure high electrical potentials (thousands of Volts) or to measure high instantaneous pressures. There are sufficiently big and pure quartz crystals in the Nature to be used for optical purposes.
There are also several forms of amorphous silica with water, such as the opal or the geyserite. From these, the black opal of Australia stands out, being one of the most valuable precious stones.