Ivory differs from bone in its finer structure and greater elasticity, and in the absence of those larger canals which carry bloodvessels through the substance of bone and appear upon it as specks or streaks according as the bone is cut lengthways to or across the grain. On examining the cross section of a tusk cut at a distance from the growing pulp, its middle is seen to be occupied by a darkish spot of different structure; this is the last remains of the pulp roughly calcined. The outer border of the tusks consists of a thick layer of cementum (commonly called "bark"), with which the whole tusk is ci lated, and the rest is ivory. The different ivories are the mammoth, found in Siberia; African, Indian, Ceylon, and Desert, found in the sands. The best ivory is African. The largest quantity comes from Africa; less than one-fourth comes from India. African ivory is closer in the grain, and has less tendency to become yellow by exposure than Indian ivory. When first cut it is semi-transparent and of a warm colour, and as it dries it becomes much lighter and more opaque. Ivory a No shrinks considerably during the drying process, so that it is necessary to season it like wood when such things as box lids are to be made from it.

In buying ivory, it is not always possible to Judge its quality before the tusk is cut up. The tusk should be smooth and polished and of a deep copper colour, and should not show any large cracks. As about one-half the length of a tusk is hollow, when cutting one up great care must be taken to cut it up to the best advantage. With age ivory turns yellow, and various recipes have been given for restoring its whiteness, but they mainly depend on the removal of the outer surface, and no more satisfactory method is known than exposing it to the light. Ivory may be made flexible by submitting it to the action of phosphoric acid; when washed and dried it becomes hard, and when moistened again resumes its flexibility - but at the sacrifice of many of its properties. Ivory takes dyes well without interfering with the subsequent polish of its surface. Of other ivories, the canine teeth of the hippopotamus furnish an ivory harder and whiter than that of the elephant and less prone to turn yellow. The tusks of the walrus furnish ivory of a dense and rather imperfect consistence.

The spirally twisted tusk of the narwhal, the teeth of the sperm whale, the ear-bones of whales, and the molar teeth of the elephant are also made use of as sources of ivory, whose quality, of course, varies greatly.