The ultimate is probably fossil ivory, which used to be a lot cheaper than it is now. Not so sure of its durability.
Elephant ivory might be a fine investment, if you can find some (legally).
Stag is about 1/2 the cost of fossil ivory and is possibly more practical as far as grip and durability.
The fine hardwoods, figured walnuts, desert ironwood, bocote, are nice. I read recently that cocobolo has been declared endangered, and may be harder to obtain in the future.
Mother of pearl is wonderful, but I have heard not always durable. I've always wondered about abalone, which has spectacular colors. I have one really large abalone shell that I might be able to make into grips, but I'm not sure I have the skill to do it.
