Rock, the solid material that makes up the earth's crust. Soil, sand, and other loose natural materials are not usually considered to be rock, although they are derived principally from rocks. Rock used for construction is commonly called stone.

Virtually all rocks are composed of one or more minerals, which are natural substances that have characteristic properties and somewhat specific chemical compositions. The largest single group of minerals in rocks of the earth's crust are the silicates, which consist of silicon, oxygen, and one or more metals. The metals found most frequently in silicates are aluminum, sodium, magnesium, calcium, and potassium.

The scientific study of rocks and their relationship to one another—and to the earth as a whole—is called geology. A geologist or other scientist who specializes in the study of rocks themselves is a petrologist; an expert in minerals is a mineralogist.

Rocks are extremely varied in their appearance and general properties. Petrologists often identify and classify rocks on the basis of three main characteristics: texture, structure, and composition. Texture refers to the shape, size, and appearance of the individual particles in rock. Smooth-textured rocks may have small, flat particles. Coarse-textured rocks may have a variety of large angular particles.

Structure refers to the overall arrangement and appearance of rock in its natural setting. For example, some rock formations occur as massive blocks or columns, while others have a pronounced layered arrangement.

Composition refers to the minerals contained in the rock. By determining the kinds and abundances of minerals in a sample of rock, a scientist can often identify the rock. The color of a rock sample is often a clue to its mineral composition; red and brown rocks, for example, may owe their color to the presence of iron-rich minerals. Density (weight of a given volume) is another aid in identifying composition; relatively dense rock may contain minerals that are rich in heavy metals such as lead.

In addition, the rock's original location, especially in relation to nearby rocks that are already known, is of help in identification and classification.

Interesting facts about rocks
Balanced Rock, in the Garden of the Gods near Colorado Springs, Colorado, is an enormous block of sandstone delicately balanced on a small base.
Bendable rock. Most rocks cannot be bent or squeezed out of shape. But thin slabs of itacolumite, a rare kind of sandstone found in India and North Carolina, can be bent by hand because of their crystalline structure.
Eight elements make up more than 98 percent of all the rocks in the world. These elements are found in about the following percentages: oxygen (46.5), silicon (27.6), aluminum (8.0), iron (5.0), calcium (3.6), sodium (2.8), potassium (2.6), and magnesium (2.0).
Floating rock. Pumice is a rock that floats on water. It was once volcanic lava filled with gases. When the gases escaped, they left millions of tiny holes that filled with air.
Rock of Gibraltar is a huge block of limestone near the southern tip of the mainland of Europe.