Calcite (CaCO3, calcium carbonate) from Manresa. Calcite is a mineral that crystallizes in several shapes, producing crystals generally white or colorless, although sometimes they can be different colors. Calcite defines the third degree of hardness of Moh's scale. It cannot be scraped by the nail, but it is easily scraped by a knife. Calcite crystals are often found coating the cracks of CaCO3 containing stones and in caves. Calcite is the main component of limestones that are used in the manufacture of cement and lime. [photo Joaquim Sanz / Museum of Geology Valentí Masachs]