Aragonite is a Carbonate mineral that can occur in conjunction with Calcium Carbonates. Aragonite differs from Calcite in the formation of the Crystal shapes which can be in lattice, crystal, columnar or fibrous in many different designs, shapes and sizes. Aragonite will form in mollusk shells, oceans and saltwater caves as inorganic precipitates called marine cements and speleotherms. Ammolite is a form of Aragonite with impurities that make it iridescent and valuable as a gemstone.