Oolite  Oolite or oölite ("egg stone") is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Ancient Greek word "ᾠόν" for egg. Strictly, oolites consist of ooids of diameter 0.25–2 millimetres; rocks composed of ooids larger than 2 mm are called pisolites. The term "oolith" can refer to oolite or individual ooids. Ooids are most commonly composed of calcium carbonate (calcite or aragonite), but can be composed of phosphate, clays, chert, dolomite or iron minerals, including hematite. Dolomitic and chert ooids are most likely the result of the replacement of the original texture in limestone. Oolitic hematite occurs at Red Mountain near Birmingham, Alabama, along with oolitic limestone. They are usually formed in warm, supersaturated, shallow, highly agitated marine water intertidal environments, though some are formed in inland lakes. The mechanism of formation starts with a small fragment of sediment acting as a 'seed', e.g., a piece of a shell. Strong intertidal currents wash the 'seeds' around on the seabed, where they accumulate layers of chemically precipitated calcite from the supersaturated water. The oolites are commonly found in large current bedding structures that resemble sand dunes. The size of the oolites reflect the time that they were exposed to the water before they were covered with later sediment. Oolites are often used in the home aquarium industry because their small grain size (0.2 to 1.22 mm) is ideal for shallow static beds and bottom covering of up to 1" in depth. Also known as "oolitic" sand, the sugar-sized round grains of this sand pass easily through the gills of gobies and other sand-sifting organisms. Importantly, this unusually smooth sand promotes the growth of bacteria, which are important biofilters in home aquaria. Because of its extremely small grain size, oolitic sand has a lot of surface area, which promotes high bacterial growth. Some exemplar oolitic limestone was formed in England during the Jurassic period, and forms the Cotswold Hills, the Isle of Portland with its famous Portland Stone, and part of the North York Moors. A particular type, Bath Stone, gives the buildings of the World Heritage City of Bath their distinctive appearance.