Ivory can be divided into broad sense and narrow sense. Ivory in a broad sense includes the teeth of some mammals, such as hippo, walrus, wild boar and whale. Ivory in a narrow sense refers to the tusks and teeth of elephants. Generally speaking, ivory refers to ivory in a narrow sense. The chemical composition of ivory includes inorganic and organic parts. Inorganic substances are mainly CaO, P2O5 and MgO, and organic substances are mainly gliadin and elastin. Ivory is generally curved and angular, with a nearly circular cross-section, characteristic lines, and a prismatic pattern composed of two groups of intersecting lines. The obtuse angle of the prism is always greater than 1 15. Fresh ivory is usually porcelain white, milky white, white and light yellow. Prehistoric ivory was blue and occasionally green. Ivory has a soft oily luster or waxy luster, with a hardness of 2.5, but excellent toughness and a relative density of 1.85. Ivory shrinks when heated; Soaking in acid will soften in a short time and decompose in a long time. Ivory has a white, pure, moist and soft beauty, which is beyond the reach of any other gem. In ancient times, it was often used as a throne by rulers, or embedded in walls and ceilings, or used as jewelry. Most of the world's high-quality ivory comes from Africa. However, with the improvement of the awareness of wildlife protection, elephants are listed as prohibited animals, so ivory materials are becoming less and less, and ivory products are becoming more and more precious.