I might be late in the news here but I just realized that the glass on the Retina MacBooks are actually plastic!
I had a chance to do whatever I wanted on a non-working screen, that my friend is replacing, and I was surprised to see deep marks left by poking/pressing my fingernails on the screen! The marks showed signs of slowly repairing themselves in a bit of time, but some seems to have remained. This is on 15-inch Retina late-2013.

I had a chance to do whatever I wanted on a non-working screen, that my friend is replacing, and I was surprised to see deep marks left by poking/pressing my fingernails on the screen! The marks showed signs of slowly repairing themselves in a bit of time, but some seems to have remained. This is on 15-inch Retina late-2013.