Ooids are tiny grains that are typically composed of CaCO3 either as calcite or aragonite. They precipitate from seawater in concentric bands around a nucleus (for instance a fragment of rock or fossil) in turbulent shallow conditions.
Once ooids form, they can accumulate and be cemented to form a sedimentary type of limestone called oolite. The above picture is a magnified and tumbled piece of oolitic chert collected fromgravel on Crowley’s Ridge in northeast Arkansas. The difference between this and typical oolite is that it came into contact with silica(SiO2)-rich ground water after it formed. The SiO2 then replaced the CaCO3 the rock was initially composed of. The polished surface provides an ideal view of the internal structure of the spherical ooids.
Office of the State Geologist Blog 