Aragonite

Aragonite is calcium carbonate arranged differently from calcite, and the difference matters more than it sounds. It is denser, harder, and — crucially — metastable at the Earth's surface, so given enough time it converts to calcite without any change in composition.

That instability is why old limestone is calcite and young shells are aragonite. Most molluscs build their shells from it, corals build reefs from it, and pearls are almost entirely aragonite. The mineral is being manufactured by living things in enormous quantity and slowly turning into something else the moment they stop.

Hard to separate from calcite by eye, and the standard tests are indirect. Aragonite is noticeably harder and denser, and it lacks calcite's perfect rhombohedral cleavage — it breaks irregularly rather than into skewed boxes.

Both fizz in dilute acid, so the acid test settles that it is a carbonate and nothing more. Habit is often the best field clue: aragonite commonly forms radiating needles, branching coral-like masses, or the twinned pseudo-hexagonal prisms found in hot spring deposits.

Aragonite precipitates from warm, shallow seawater and from hot springs, and it is the form biology overwhelmingly prefers: shells, coral skeletons and pearls are aragonite rather than calcite because organisms control which polymorph they grow.

At surface temperatures and pressures calcite is the stable form, so aragonite converts to it over geological time. That conversion is why aragonite is common in recent sediments and rare in ancient limestone, and why a fossil shell's original aragonite has usually been replaced.

Little is quarried for its own sake. Its economic importance is as the raw material biology supplies: nacre for buttons, inlay and jewellery, and cultured pearls, which are aragonite deposited by an oyster around an implanted nucleus.

Aragonite sand is used in aquaria and to raise the alkalinity of water, and crushed shell — largely aragonite — is fed to poultry as a calcium supplement. Industrially the demand is met by calcite and limestone, which are far more abundant and chemically identical.