Originally they were made from wood and then clay. It evoled to the point where billiard balls were made from ivory. One would think a fear for endagering elephants would have prompted steps to be taken to find a subsitute material. Not so, it was a concern for the elephant hunter that prompted the development of a substitute, celluloid. Today they are made from a combined plastic and resin because of the ease of molding.
Billiard balls have been made from many different substances over the centuries. There have been wooden and metal balls, as well as clay and plastic. The most popular balls were made from Ivory, the kind that comes from elephant tusks. You can find more information here: http://en.wikipedia.org/wiki/Billiard_ball#History