I believe the VF imitation ivory is either polyester or polyurethane resin.
He has two versions, "grade 1" and "grade 2" - there might be a difference in machinability between them, I've only tried one (I forget which it was) and it was not at all easy to cut clean threads in it with tap & die. It might also change from batch to batch unless the manufacture has really high quality control measures in place.
A couple of years back I tried a product called Elforyn Super Tusk - but the place I bought it from has discontinued sales because of variable quality issues -
read all about it here.