Fiberglass for nails is on the same lines of fiberglass for boats and cars..but not as thick
Why it goes transparent I guess is because it isn't a solid material, but a very fine thread woven tightly, when the resin touches it, the fiberglass absorbs the resin. Hmmm, I think that is a confusing answer eh?
Your best bet is to ask your instructor why it goes clear when resin is applied. I personally never thought about it.