A pisolite is a sedimentary rock formed from pisoliths. These are concretions - often of calcium carbonate, but commonly in rarer minerals - which resemble ooids but are always more than 2 mm in diameter. These grains are approximately spherical and have concentric layers reaching 10 mm in diameter. The name derives from the Hellenic word for pea. Bauxites, limonites and siderites often have a pisolitic structure.… (More on Pisolite)