One of the two main psuedomorphs of calcium carbonate. Both aragonite and calcite can be precipitated in the earth's ocean but they generally don't form at the same time. Calcite is precipitated in oceans with a low manganese content. This occurs during times when there is a lot of seafloor spreading. Aragonite forms when there are large amounts of manganese available in the ocean. The earth has switched between oceans with these kinds of chemistry main times. Currently we have an aragonite ocean, but the ocean was calcitic during the cretaceous.
Animals with shelly parts use calcium carbonate from the seawater to make their shells. The kind of shell hey make is dependant on the chemistry of the ocean at the time. Animals which evolved in calcitic seas have calcite shells, while animals which evolved in aragonite seas have aragonite shells. Most modern seashells are aragonite.
Aragonite is metastable -- over long periods of time things that have been ade from aragonite tend to calcite.