Silcrete is a duricrust formed when quartz grains are cemented together by dissolved silica. In fine-grained silcrete, quartz sand grains appear to be ‘floating’ in the matrix of silica, while other varieties are composed mostly of quartz grains. The latter varieties are a form of orthoquartzite, or silicified sandstone. Silcrete is a very common source of stone for tools made by Aboriginal people in Australia, but it can also be found in other parts of the world, such as South Africa and the Congo Basin. The flaking quality of silcrete can often be improved by heat treatment.