Marble is a soft metamorphic rock that forms at the edges of tectonic plates, where limestone is exposed to regional metamorphism. The heat and pressure of the metamorphic zone force calcite inside the limestone to recrystallize and merge into the classic marble consistency.
Marble begins as limestone near the edges of tectonic plates. As subduction creates high pressures and temperatures around the limestone, the calcite and other minerals inside the stone begin a metamorphic transformation. Fossils and other organic debris trapped in the limestone break down at the molecular level and become reconstituted into small crystals of calcite. The calcite crystals expand as metamorphic conditions persist, eventually reaching several millimeters in diameter. As they grow, the crystals interlock to form a stable rock matrix that holds the marble together. This process substitutes the foliation, or peeling process, common to other metamorphic stone.
Another type of marble called "dolomite marble" forms when dolomite is exposed to contact metamorphism, which can take place far from a plate boundary. This process is similar to the formation of normal marble, but it often occurs in proximity to hot granitic intrusions. As a result, dolomite marble rarely forms the extensive deposits typical of most marble.