A pisolite is a sedimentary rock formed from pisoliths. These are concretions - often of calcium carbonate, but commonly in rarer minerals - which resemble ooids.These grains are approximately spherical, and comprise concentric layers reaching 5-8 mm in diameter. The name derives from the Hellenic word for pea. Bauxites, limonites and siderites often have a pisolitic structure.… (More on Pisolite)