What is Marble?

According to Geology.com, Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism.

Metamorphism is the change of minerals or the distinct arrangement of minerals ( geologic texture) in pre existing rocks, primarily due to heat, pressure and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change, even though the rock remains a solid.

Marble 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.