Aragonite

Chemically identical to calcite but structurally distinct, aragonite is the orthorhombic polymorph of calcium carbonate (CaCO3). It is metastable at the Earth's surface and inverts slowly to calcite over geological time. The mineral is transparent to translucent with a vitreous luster, a white streak, and poor {010} cleavage; crystals commonly form prismatic, acicular, or pseudo-hexagonal twinned shapes. Its hardness ranges from 3.5 to 4 and its specific gravity from 2.93 to 2.95. Aragonite carries particular biological significance as the carbonate that composes nacre, or mother-of-pearl, and the bulk of natural pearls, which mollusks secrete in preference to the more stable calcite.

Aragonite is the high-pressure polymorph of calcium carbonate. It is less stable than calcite at Earth's surface conditions and will convert to calcite over geological time. Mother of pearl and most pearls are composed of aragonite.