Dicho got it right. Ivory is one material that can survive for a very long time without changing its composition. The correct term is "fossil ivory," which, in this case, means ivory old enough not to be covered by endangered species laws.
Mammoths/Mastodons actually existed recently enough that it is rare to find fully fossilized remains of any kind. The bones are porous enough to have undergone some replacement mineralization, but a fossil, in the archaeological sense of "a rock in the shape of the bone that was there" is very rare.
If the material was actually fossilized (fossilised for the Brits) it would mostly resemble whatever rock is common in the area - and unless you thing that a set of chalk or granite pegs would be a good thing, would be useless on a guitar.