Aragonite

Aragonite, carbonate of lime crystallising in the prismatic system, and rather harder and heavier than the more common form calcite. Aragonite often occurs in twin-crystals forming short hexagonal prisms with grooved sides or six-rayed stars, or in a coral-like stalactitic form known as flos ferri from being associated with iron ores. Like calcite, it effervesces freely with acid; but it is deposited from hot solutions.