Minerals are the most common solid materials found on Earth. All Earth's lands and oceans rest on a layer of rock made of minerals. Rocks on Earth's surface are also made of minerals. Soil contains tiny pieces of minerals broken from rocks. Minerals are also found on our moon and on Mercury, Venus, and Mars. Image to left: Gold is a mineral that is a metal. Credit: World Book photo, Field Museum of Natural History, Chicago Minerals include such common substances as rock salt and pencil "lead" and such rare substances as gold, silver, and gems. There are about 3,000 kinds of minerals. But only about 100 kinds are common. Most of the others are harder to find than gold.

Minerals vary greatly in the way they look and feel. Some minerals have a glassy surface that sparkles with color. Others look dull and feel greasy. The hardest minerals can scratch glass. The softest minerals can be scratched by a fingernail. Photo of a piece of azurite that has different shades of blue Minerals have several features that make them different from other substances. First, minerals are found in nature. They are not made by people. Next, each particular kind of mineral is made of the same materials, no matter where it is found. Third, the atoms, or building blocks, of minerals are arranged in a set pattern and form solid units called crystals. Finally, almost all minerals are made up of substances that were never alive.

Image to right: Azurite is a blue mineral. Credit: World Book photo, Field Museum of Natural History, Chicago How to cite this article: To cite this article, World Book recommends the following format: "Mineral." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.