Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. It is composed primarily of the mineral calcite (CaCO3) and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite. Under the conditions of metamorphism, the calcite in the limestone recrystallizes to form a rock that is a mass of interlocking calcite crystals. A related rock, dolomitic marble, is produced when dolostone is subjected to heat and pressure.Photo Gallery: The Many Uses of Marblea
Ruby in Marble: Marble is often the host rock for corundum, spinel, and other gem minerals. This specimen is a piece of white marble with a large red ruby crystal from Afghanistan. Specimen is about 1 1/4 inches across (about 3 centimeters). Specimen and photo by Arkenstone / www.iRocks.com.How Does Marble Form?
Most marble forms at convergent plate boundaries where large areas of Earth’s crust are exposed to regional metamorphism. Some marble also forms by contact metamorphism when a hot magma body heats adjacent limestone or dolostone.Before metamorphism, the calcite in the limestone is often in the form of lithified fossil material and biological debris. During metamorphism, this calcite recrystallizes and the texture of the rock changes. In the early stages of the limestone-to-marble transformation, the calcite crystals in the rock are very small. In a freshly-broken hand specimen, they might only be recognized as a sugary sparkle of light reflecting from their tiny cleavage faces when the rock is played in the light.