I got curious. Apparently they are from cow bones and are processed at high temps to avoid immune rejection.

Xenogenic bone is derived from non-living bone of another species, usually a cow. The bone is processed at very high temperatures to avoid the potential for immune rejection and contamination. Like allogenic grafts, xenogenic grafts serve as a framework for bone from the surrounding area to grow and fill the void. Source

There are different kinds of grafts though that do include cadaver or auto-grafting, but I am 99% sure my doctor said mine was cow bone.