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