Marble is a metamorphic rock formed when limestone is exposed to high temperatures and pressures. Marble forms under such conditions because the calcite forming the limestone recrystallizes forming a denser rock consisting of roughly equigranular Calcite crystals. The variety of colours exhibited by marble are a consequence of minor amounts of impurities being incorporated with the calcite during metamorphism. While Marble can appear superficially similar to Quartzite, a piece of marble will be able to be scratched by a metal blade, and marble will fizz on contact with dilute Hydrochloric Acid.