There are three types of rocks: igneous, metamorphic, and sedimentary, in order of abundance. Within these classifications are many hundreds of types. The first two are formed under conditions of extreme heat and pressure. Scientists now know enough about rocks to produce some artificially — for example ruby and diamond.
Igneous rocks are formed when magma cools into solid form. This can happen on the surface with volcanic discharge, but primarily takes place beneath the earth’s crust. Over 700 varieties of igneous rock have been described, some with crystals and some not. Igneous is derived from the Latin word for "fire," ignis. A majority — about 90% — of igneous rocks are silicate minerals, which are rich in silicon and oxygen. Quartz is among the most familiar and abundant of this type; clays and feldspar are other examples.
Metamorphic rocks are formed when a preexisting rock, called a protolith, is under conditions of high heat and pressure, causing it to metamorphose chemically, structurally, or both. The protolith might be an igneous, sedimentary, or another metamorphic rock. Slate, marble, and quartzite are some examples of this type. Most have a structural characteristic called foliation, which means that the rock is composed of many tiny compressed layers. The process of metamorphosis is usually accompanied by complex chemical reactions.
Sedimentary rocks are less abundant than the other two varieties, composing only 5% or so of the earth’s crust. They are formed in one of three ways: when bits of a larger rock chip off and settle to the ground, when the remains of plants or animals build up in quantity, or when a solution containing a mineral leaves deposits over time. They are named for their origin — sediment.
This type of rock is formed over long periods of time as tiny grains of material are pressed against each other and join loosely. The process by which sedimentary rocks are formed is delicate enough that fossils can be preserved within them. Common examples include sandstone, chalk, and limestone.