The other evening I mixed too much concrete to make the leaf I wanted for a workshop. It was getting late and I didn't want to do another leaf so, in the interest of science, I plunked the concrete in a plastic grocery sack, smooshed a shallow depression in it and with gloved hand, pushed some broken auto glass into it...black and clear. When the concrete was set, I "unmolded" my creation, cleaned the surface with Limeaway, rinsed well and was pleased that the glass sparkled in the sun. Only problem was that, in looking at the whole thing, it resembled, well...diamonds in a cow pie. Incidentally, if ever I try this again, I'll use only the clear glass. It had more sparkle.