An animal’s would do in a pinch, but for real power, it would need to be human. I still can’t help but to find it curious. What they give away, what is no longer of any use to them is a treasure to us. Because we, of course, know how to use it.
The roots. The bone. The blood that once pulsed within. The essence of strength and health and prosperity.
It must be undamaged, unspoiled. A broken tooth or a rotten one has broken or rotten magic in it. It must be freely given. Not found, or there is too little magic in it. Not taken by force. Or else there is no magic in it at all.