My Blasting grit has them (thin strands of glass). Smaller diameter than what is seen in that picture. Be aware of them while you rinse it so as to not stick yourself too much. I have only had a few penetrate my skin and I rinsed mine really well for quite a while before placing it in my tanks.
That being said, my Oto cats rest on the blasting grit often and my cories (pygmaeus, paleatus) have no issues rooting aound in it all day long along with Malaysian Trumpet snails.