Marble is a metamorphic rock (the transformation of an existing rock) composed of re-crystallized carbonate minerals, most commonly calcite or dolomite, which is basically subjected to heat and pressure (temperatures greater than 150 to 200 °C and pressures of 1500 bars) causing profound physical and/or chemical change. The characteristic swirls and veins of many colored marble varieties are usually due to various mineral impurities such as clay, silt, sand, iron oxides, or chert which were originally present as grains or layers in the limestone.