Minerals are natural solids formed through many geological processes under high pressures. To be thought as a true mineral, a substance must have a crystal structure with uniform physical things throughout. Minerals differ in composition from pure elements and simple salts to very complex silicates with thousands of known forms. In contrast, a rock sample is a random aggregate of minerals and/or mineraloids, and has no specific chemical composition. Most of the rocks of the Earth's crust have quartz (crystalline SiO2), feldspar, mica, chlorite, kaolin, calcite, epidote, olivine, augite, hornblende, magnetite, hematite, limonite and a few other minerals. Some minerals, like quartz, mica or feldspar are common, while others have been found in only a few places in the world. The largest group of minerals by far is the silicates (most rocks are ≥95% silicates), which are made largely of silicon and oxygen, also with ions of aluminium, magnesium, iron, calcium and other metals.