Michael's & JoAnne's sells kits similar to what you are talking about. Ask where the concrete stepping stone kits are & you can see what they are like. Often once you get the kit you do your own thing. They come with stained or colored glass pieces. I broke a couple of dishes to add to mine when I made it. I wanted a dragonfly so found a pic & freehand made it size I needed & put all the pieces of glass on it the way I wanted. Then I put the pieces nice side down in the mold & spooned the concrete in on top, thought I was careful, well I got an uneven top. I do better filling the form with concrete & laying the pieces on top & pushing down into the wet concrete. Anyway I didn't throw the thing out but put in garden, after couple of years of walking on it, washing it off,etc The pattern shows better & so what if little dirt in low spots, just looks older(antique) Only thing wrong with placing stones or glass on top, the edge is not rounded so much. Might be able to correct that by rubbing the new stone on edges about a week after you make it with another piece of concrete to get a smoother more professional edge. I don't see a problem unless your GK's would be walking on them barefoot! Kits are around $20 but with 40% off coupon are pretty reasonable. If there aren't too many sharp edges you might be able to get them smoothed with an old nailfile, kind that is paper& foam in center, about an inch wide & several inches long., Might work with just a real old metal file also but might scratch the glass. Same thing you would do if you best goblet got a tiny chip in rim, smooth it out. Jan