Ivory and bon are both cut very thin. A lot thinner than any of the plastic scales I've seen. If you'd got access to a strong magnifying glass or a jewellers loupe, have a look at them. Bone is pourous and you can see lots of very small holes in it. When bone scales are dirty, they look like they have black flecks on them. This is dirt in the hole. Under magnification, ivory has a clear grain to it. Also if you magnificatin is high enough, ivory looks to have little hair follicles in it.

I hope that helps.