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Rocks are naturally formed, non-living Earth materials. Rocks are made of collections of mineral grains that are held together in a hard, solid mass. The mineral grains in rocks may be so tiny that you can see them only with a microscope, or they may be as big as your finger. Rocks are identified primarily by the minerals they contain and by their texture. Each type of rock has distinctive minerals. A rock may be made of grains of all one mineral type. Much more commonly, a rock is made of a mixture of different minerals. Texture is a description of the size, shape, and arrangement of mineral grains. Two rocks may have the same chemical composition and contain mostly the same minerals, but they may have different textures. Their different textures indicate different histories. For example, a rock that formed from molten rock will have small grains and a fine texture if it cooled quickly on Earth’s surface. A rock with the same composition that cooled slowly under Earth’s surface will have larger grains and a coarser texture.
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