Sample of crested barite from Missouri
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Spar is a term that is used to refer to a category of crystals that have readily discernible faces. Crystallized, earthy, and some metallic, a spar will easily break into rhomboidal, cubical, or laminated fragments with polished surfaces, but without regard to the ingredients of which they are composed. Amongst miners the term is frequently used alone to express any bright crystalline substance. Most frequently, spar describes easily cleaved, lightly colored nonmetallic minerals such as feldspar, calcite or barite. Barite (BaSO4), the main source of barium, is also called "heavy spar" (Greek "barys" means "heavy"). The largest type of spar is of the phreatic variety, since it essentially much more time to grow than average spar. It grows best where the water is just barely saturated with minerals. Some caves are like giant geodes, the walls and ceilings completely lined with spar. Jewel Cave and others in the Black Hills of South Dakota contain large quantities of spar.
Sometimes, spar will form in the air due to solutions seeping out of the cave's walls or through porous sediments. When grown in the air, it is often made of gypsum or selenite. Sometimes it will form as small needles found in sediments. Others spar can be found on the tips of gypsum chandeliers.
Most pool spar is nicknamed "dogtooth" because it comes to a jagged point. Other pool spar, especially ones formed along the flat surface of the pool, may even form in a triangular shape.