... "comparison approach" with the help of older and established price lists both online and hardcopy. Though I am not in that business, I have put values on some noted collections when traded hands, but that material was mostly vintage European MOP and early-Celluloid types where descriptions carried photos matched with actual maintained collections.

Regarding insurance; of which I can not help, I do know for a fact that my initial costs of the few Ivories I had at home was hit with a "depreciation factor" (homeowners) as personal items for some reason that I didn't bother to look into at the time... where market value was much higher... and of course this was material purchased at high-end levels as well.