Aragonite

Aragonite was named by Abrahan Gottlieb Werner after Molina de Aragón, Spain, where this mineral was first described.

Aragonite forms from calcium carbonate, like Calcite. However, Aragonite crystallizes in orthorhombic crystals; whereas Calcite crystallizes in trigonal crystals. Most large Aragonite crystals are twinned growths of three individual crystals that create pseudo-hexagonal trillings. Trilling can be identified by their multi-directional basal striations from each individual member crystal. 

Some Aragonite crystals are actually Calcite pseudo-morphs after Aragonite. Whereas, when Aragonite is heated it can turn into Calcite. Aragonite begins to convert to calcite at about 400°C when heated in dry air. This is called thermoluminescent.

Fun FYI: Mollusks and related invertebrates secrete Aragonite creating the iridescence of Pearls and Mother-of-Pearl.

Aragonite may also contain sand inclusions, which gives it a brown color. Blues, pinks and other colors are from other minerals present during its creation.