Ivory carving is the carving of ivory, that is to say animal tooth or tusk, generally by using sharp cutting tools, either mechanically or manually. Objects carved in ivory are often called "ivories". Humans have ornamentally carved ivory since prehistoric times, though until the 19th-century opening-up of the interior of Africa, it was usually a rare and expensive material used for small luxury products. Very fine detail can be achieved, and as the material, unlike precious metals, has no bullion value and usually cannot easily be recycled, the survival rate for ivory pieces is much higher than for those in other materials. Ivory carving has a special importance to the medieval art of Europe because of this, and in particular for Byzantine art as so little monumental sculpture was produced or has survived. As the elephant and other ivory-producing species have become endangered, largely because of hunting for ivory, CITES and national legislation in most countries have reduced the modern production of carved ivory. The material Ivory is by no means exclusively obtained from elephants; any animal tooth or tusk used as a material for carving may be termed "ivory", though the species is usually added, and a great number of different species with tusks or large teeth have been used. Teeth have three elements: the outer dental enamel, then the main body of dentine, and the inner root of osteo-dentine. For the purposes of carving the last two are in most animals both usable, but the harder enamel may be too hard to carve, and require removal by grinding first. This is the case with hippopotamus for example, whose tooth enamel (on the largest teeth) is about as hard as jade. Elephant ivory, as well as coming in the largest pieces, is relatively soft and even, and an ideal material for carving. The species of animal from which ivory comes can usually be determined by examination under ultra-violet light, where different types show different colours. Eurasian elephant ivory was usually obtained from the tusks of elephants in India, and in Roman times, from North Africa; from the 18th century sub-Saharan Africa became the main source. Ivory harvesting led to the extinction, or near-extinction of elephants in much of their former range. In early medieval Northern Europe, walrus ivory was traded south from as far away as Norse Greenland to Scandinavia, southern England and northern France and Germany. In Siberia