Limestone is a sedimentary rock formed from different crystal forms of calcium carbonate and is an important building stone. Most limestone contains fragments of marine organisms, seashells and sediment, which have been compressed to form solid rock. We have a wide range of limestone, many with quite different characteristics, colours and markings and it is not uncommon to see fossilised shells or skeletal fragments, fissures and quartz veins or occasional surface pitting or resin fill.