Rock that is mainly made of calcite, a mineral form of calcium carbonate, is known as limestone. It forms when the carbonate crystallises out of solution. This can evolve in many different ways, leaving behind a different kind of limestone every time.

Most limestone is typically grey; however, almost every other colour has been found over time.

Limestone can form almost completely without the presence of acid from organisms. It is simply forced out of solution, by evaporation. More often, though, limestone is formed by the work of organisms, such as aquatic ones.

Limestone makes an excellent building material because it can be easily worked. It can even be used for the purification of waste water and gases! Most limestone tends to be of fairly uniform tonality but, as always, the exception confirms the rule - some limestones have strong markings, such as blotches or strong veins.