In commerce, ivory is classified as live (from recently killed animals) and dead (tusks long stored or on the ground for extended periods and lacking the resilience of live ivory). Ivory may be of a soft or hard variety; the former type is more moist, cracks less easily than the brittle hard ivory, and is easier to work. In the West, soft ivory, obtainable primarily from the eastern half of Africa, was preferred to the hard variety from W Africa. Green, or guinea, ivory denotes certain types of ivory obtained from a wide belt in north central Africa, from the east to the west coasts. At various periods in Africa, native peoples, Arabs, and European colonial powers dominated the trade (now banned) in ivory. Zanzibar, Antwerp, London, and Hong Kong have been major centers of ivory commerce.
Natural substitutes (e.g., tagua, or vegetable ivory) for ivory or near equivalents have long been used. The tooth structure of many other animals, such as the hippopotamus, walrus, narwhal, sperm whale, and wild boar, is also often called ivory.