Polarised light micrograph of oolitic limestone chert of the dolomite type from Calw, Germany. Oolites are small (5 to 3 millimetre in diameter), spherical grains composed of calcium carbonate or other carbonates. Oolites form by the gentle rolling to and fro of small grains, such as sand or shell fragments, by wave action on the bottom of warm shallow marine waters. These grains act as nuclei for the precipitation of calcite in thin concentric layers around them. The size of the oolites reflect the time that they were exposed to the water before they were covered with later sediment. The chert layer may contain small silicified mussels and snails. The age of this shell limestone chert is 238 million years. It is very hard, as the ooids are largely bound in pebbles. Magnification: x30 when printed at 15 centimetres wide.