Dolostone or dolomite rock is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite, CaMg(CO3)2. Most dolostones formed as a magnesium replacement of limestone or lime mud prior to lithification. It is resistant to erosion & less soluble than limestone in weakly acidic groundwater. Technically, dolostone has a stoichiometric ratio (reaction & product ratio) of nearly equal amounts of magnesium and calcium. This particular example comes from the Clitheroe area of Lancashire. Specimens are supplied in labelled card trays.