Marble forms when limestone is recrystallized by heat and pressure.
Limestone is commonly rich in calcite, a carbonate mineral. During metamorphism, heat, pressure and chemically active fluids allow the original calcite grains to recrystallize into a new interlocking texture. The resulting rock is marble, even though its chemical composition may remain broadly similar to that of the starting limestone.
Pure calcite marble is usually white, but impurities can create gray, green, pink, brown or black patterns. Fossils and bedding in the original limestone may be distorted or erased as crystals grow. This recrystallization distinguishes marble from ordinary limestone, which retains its original sedimentary texture.
Marble has been used in sculpture, buildings and polished surfaces since antiquity. Carrara marble from Italy is especially famous, and Michelangelo used varieties from the Carrara region. Geologists also use the term marble for metamorphosed carbonate rocks containing dolomite, not only for rocks derived from pure limestone.