Fossilized 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 woolly mammoth, mastodon, and walrus ivory. Mammoth/mastodon ivory is at least 10,000 years old and does indeed come from those very large, furry (and extinct) ancestors of elephants. The characteristics of mammoth and mastodon ivory are similar to elephant, with the added bonus that no animals were harmed to get it. Perhaps ancient man hunted some of the previous owners of this ivory, but I like to think they did that in order to eat. Fossil mammoth and mastodon ivory has a distinct crisscross pattern of fine lines which show the growth of the tusks. These lines help me to ensure that the ivory I buy is definitely ancient ancient ivory and not elephant in disguise; elephant and mammoth/mastodon ivory have distinct angles to their line patterns. I check every piece. Because of its age, mammoth ivory is rare and sometimes more difficult to work with, but it yields lovely, lustrous results.
Walrus ivory, ancient and current, has a totally different story. Fossilized walrus ivory can be several hundred or several thousand years old. To protect the current walrus populations, fossil walrus is a highly regulated material, sourced only from Alaska, and can only be sold within the US. Fossil walrus ivory has a unique look because walrus tusks are essentially big teeth, so they have an outer enamel layer and an inner pulp layer. The outer layer is smooth with little texture or variation (unless minerals have given it stripes!), but the inner layer is swirled like burled wood. The two layers often absorb minerals very differently, with striking results.