Calcite, calcium carbonate, is one of the most ubiquitous minerals on the earth's surface. It is the constituent of seashells and tiny marine organisms. When these animals die their shells fall to the seabed and eventually are compressed and transformed into the rock known as limestone. Calcite readily dissolves in surface waters and is redeposited as cave formations. Calcite is dimorphous (same chemistry, different crystal structure) with aragonite, the constituent of pearls. Both minerals easily form crystals and present a dazzling array of crystal forms (shapes), from microscopic to enormous size. Calcite crystals are common and abundant in many types of mines where minerals have deposited from hot solutions. Good examples are found in the so-called tri-state lead-zinc mining district of Missouri, occurring mainly with the sulfides galena and sphalerite. The St. Joe Lead District is one part of this vast zone of mineralization.