Ah, that's not so good, I have to rely on subtle bumpmapping. If that is really what Matt means. I didn't use any glass shader, btw, just the default, etc.
I's a ZBrush transformed DAZ Genesis model; just divided the head a few times extra and added some wrinkles.
It's not displacement, WAS, it's bump, as the object is raytraced. But maybe you do mean that.