Bonded ivory is basically epoxy plastic with real ivory dust mixed in. What kind of ivory and how much, I have no idea. They are solid and heavier than regular plastic. As mentioned, they feel like the real thing.
Mine are not an exact fit, but they can be sanded, polished, and "aged" - or so they say.
Altamont offers a lot of different style grip panels & materials for different guns.
(disclaimer - I have no affiliation whatsoever with the company - this was my first purchase from them)