(1) Often primary constituent of the shells of marine organisms, e.g., plankton.. (2) Commonly found around hot springs and caverns as stalactites and stalagmites. (3) Calcite seas existed in Earth history when the primary inorganic precipitate of calcium carbonate in marine waters was low-magnesium calcite, known as the Ordovician and Jurassic periods.
High-grade optical calcite was used in World War II for gun sights, specifically in bomb sights and anti-aircraft weaponry. Also, experiments have been conducted to use calcite for a cloak of invisibility.
Trilobites, which became extinct a quarter billion years ago, had unique compound eyes that used clear calcite crystals to form the lenses.