Fossil ivory refers to ivory that has been in the earth for hundreds or thousands of years. The ivory has not quite turned into rock, but has taken the first step. Minerals from the soil have seeped into the ivory over time, imparting unique colors and visual textures. The result is that the "fossilized" ivory remains lightweight but gains fantastic browns, corals, tans—even blues and greens!
Fossil ivory, known also as "ancient ivory," comes from several different animals. I use wooly mammoth/mastodon and walrus ivory.