Limestone is a carbonate sedimentary rock mainly composed of calcite and sometimes dolomite, clay minerals and clastic minerals. It can be grey, greyish black, greyish white, pale red or reddish brown. It is usually soft and will fizz in diluted hydrochloric acid. Limestone has a complex structure, and there are two main kinds of structure: clastic and crystalline grain structures. Because limestone is soluble, it often forms graceful landscapes consisting of various forms, such as stone forests, peak clusters, peak forests, natural bridges, sinkholes, and karst caverns. The extensive limestone landforms in Guizhou, Guangxi, Yunnan and Hunan Provinces have become valuable tourism resources in China.