Rocks, also referred to as Rock (singular English) are hard, but somewhat brittle molecular objects containg crystals of one or more minerals. There are many varieties of rocks including basalt and conglomerate, but many simply know them as rocks.
Rocks are classifed as one of three types: sedimentary, igneous and metamorphic. Sedimentary rocks (for instance, conglomerate, sandstone, shale, and limestone) are formed when particles such as sand, clay, or shells are deposited, compacted, and buried. Igneous rocks (basalt, andesite, dacite, rhyolite) are formed when magma is extruded by a volcano or at spreading undersea ridges such as the Mid-Atlantic Ridge. Obsidian, though formed from magma, is not technically a rock because it lacks crystal structure; it is, in fact, a natural glass. Metamorphic rocks (marble, slate) are formed when igneous or sedimentary rocks are buried and subjected to sufficient heat and pressure.
There are many types of rocks other than geological rocks that are found in the ground. These include candy rocks, a form of hard sugar crystal; crack rocks, an addictive substance derived from cocaine; and Hot Rocks, a Rolling Stones CD.