Marble from the Miocene of Colorado, USA. (6.2 cm across at its widest)

Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy.

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).

The specimen shown above is from the Yule Marble, the state rock of Colorado, quarried near the town of Marble, Colorado, USA. Yule Marble formed by contact metamorphism of the Leadville Limestone (Mississippian). The igneous intrusion that metamorphosed limestone into marble is the Treasure Mountain Granite (Middle Miocene, 12 Ma).

Geologic unit & age: Yule Marble, Middle Miocene, 12 Ma