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 minerals have seeped into the ivory over time, imparting unique colors and visual textures. The ivory remains lightweight and sculptable 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 (or mastodon) and walrus ivory. Mammoth 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 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 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 mammoth ivory and not elephant in disguise; elephant and mammoth ivory have distinct angles to their line patterns. I check every piece. Because of its age, mammoth ivory is more rare and more difficult to work with, but it yields lovely, lustrous results.
Walrus ivory, ancient and current, has a totally different story. Fossil walrus ivory can be several hundred or thousand years old. To protect the current walrus populations, fossil walrus is a highly regulated material, sourced only from Alaska and only saleable within the US. Fossil walrus ivory has a unique look. 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.