Barite. Source: Mineralogical Information Institute.
Barite, which is also called baryte, barytes, and heavy spar, is the chief ore of barium. It consists of barium sulfate (BaSO4) in the form of colorless or white (when pure), brown, yellow, or gray orthorhombic crystals with perfect cleavage in three dimensions. Barite is a very dense mineral (its name comes from the Greek baros meaning "heavy") often found in association with lead or zinc ores. Because of its high density it is used to make muds for lubricating and cooling oil-well drilling equipment. It is also used in the chemical industry for paper-making, rubber manufacture, and high-quality paints. Hardness 3–3.5, relative density 4.5.