Aragonite is a carbonate mineral consisting of calcium carbonate (CaCO₃). It is formed under special conditions, generally in caverns and hot springs. Present day mollusc shells are made of aragonite. It readily converts to calcite (another mineral form of calcium carbonates). It occurs as orthorhombic system groups of needle-like crystals or massive deposits. Its appearance is glassy white. Hardness 3.5–4, specific gravity 2.9.