Wikipedia defines it:
Aragonite is a carbonate mineral. It and the mineral calcite are the two common, naturally occurring polymorphs of calcium carbonate, CaCO3. The crystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, an orthorhombic system with acicular crystals. By repeated twinning pseudo-hexagonal forms result. It may be columnar or fibrous, occasionally in branching stalactitic forms called flos-ferri (flowers of iron) from their association with the ores at the Carthinian iron mines. The type location for aragonite is Molina de Aragón (Guadalajara, Spain), 25 km outside Aragon. A whole aragonite cave (the Ochtiná Aragonite Cave) is situated in Slovakia. In the USA, stalactitic aragonite is known from Carlsbad Caverns.
Aragonite forms naturally in almost all mollusk shells. Because the mineral deposition in mollusk shells is strongly biologically controlled, some crystal forms are distinctively different from those of inorganic aragonite. In some mollusks, the entire shell is aragonite; in others, aragonite forms only discrete parts of a bimineralic shell (aragonite plus calcite). Aragonite also forms in the ocean and in caves as inorganic precipitates called marine cements and speleothems, respectively. The nacreous layer of the aragonite fossil shells of some extinct ammonites forms an iridescent material called ammolite. Ammolite is primarily aragonite with impurities that make it iridescent and valuable as a gemstone. Aragonite is thermodynamically unstable at standard temperature and pressure, and tends to alter to calcite on scales of 107 to 108 years. The young age of the California blueschists has been famously demonstrated by the finding therein of aragonite not yet reverted to calcite.
We define it:
Aragonite is the material of choice for use as buffer in marine or cichlid aquariums. which counteracts any pH fluctuation that causes deviation from 8.2. the prime pH value of marine water in the natural coral reefs. Thus. aquariums with sand bed layer composed of Tropic Eden™ 's marine aragonite significant differ from aquariums that use relatively inert calcium carbonate materials or the calcite form of coral skeleton that usually stabilize at an exceedingly low pH value of 7.5 - 7.8. A highly unfavorable pH value to marine fishes, and all coral reef organisms.
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