The forces of weather on the surface of the Earth can change the minerals that make up rocks. When mechanical weathering processes destroy old rocks, softer silicate minerals such as olivine and plagioclase may dissolve away, leaving behind harder minerals, such as quartz. In chemical weathering, the atoms react with oxygen or water in the environment. (Think of iron exposed for several months to the water and air outdoors. The iron rusts, and rusting is a kind of chemical weathering.) In the presence of water, some minerals transform into more hydrous minerals. Other minerals, such as calcite (calcium carbonate), dissolve completely in water over time. Some silicate minerals, such as quartz and garnet, are more resistant to weathering than other silicates, such as feldspar and mica.

Polymorphic minerals have the same chemical composition but different crystal structures. (The word "polymorphic" means "many forms" in Greek.) These structures may have vastly different properties. Polymorphism occurs because the minerals crystallized under different temperature and pressure conditions. The most well-known example of polymorphism is carbon: it occurs both as graphite - a soft, opaque mineral that splits into layers - and diamond - transparent and the hardest substance on Earth. Silicon dioxide has several polymorphs: quartz, tridymite, cristobalite, coesite, stishovite, and seifertite. The last three of these polymorphs form only under extreme conditions of temperature and pressure - in meteorite impacts in nature or experiments in the laboratory.