Calcite sample, close-up. This is oolitic calcite. Calcite is a general term for calcium carbonate minerals. Limestone is mostly calcite. An ooid is a nearly perfectly circular calcite deposition formed from sediments on the seabed under the action of moving waters in a shallow sea. Rocks consisting of compressed ooids are called oolites. This sample is from the Jurassic calcite sediments of central Switzerland.