Marble is a common, crystalline-textured metamorphic rock composed of calcite (CaCO3 - calcium carbonate). It forms by intermediate- to high-grade metamorphism of limestone. Marble varies in color and crystal size, but is reliably identified by its crystalline texture, by bubbling in acid, and by not scratching glass (marble has a hardness of 3, while glass has a hardness of 5.5).
Marble (7.6 cm across) from the Murphy Marble Belt near Tate, southern Pickens County, northern Georgia, USA. This marble was originally a Cambrian limestone that was regionally metamorphosed during the Ordovician Taconic Orogeny.
Marble - part of a large block of Yule Marble, the state rock of Colorado, from near the town of Marble, northern Gunnison County, western Colorado, USA. Yule Marble formed by contact metamorphism of the Leadville Limestone (Mississippian). The igneous intrusion that metamorphosed limestone into the Yule Marble is the Treasure Mountain Granite (Middle Miocene, 12 m.y.). (CSM public display, grounds of the Colorado School of Mines Geology Museum, Golden, Colorado, USA)