I think they would hold up a lot better than most people realize when used as projectiles.
Ah but not all small, glass spheres or marbles are the same. I have a Chinese Checkers set that uses glass "marbles" and if you take two or three, and squeeze them inside your fist while rubbing them together, they will flake off layers of colored glass. So, marbles made for the game of "marbles" yes, but glass spheres that look like marbles, maybe not.
LD