Mine was a little crumbly after I used a hammer to break it up. Mine did not have many sharp edges. If you are concerned put a towel over the rock and break off the sharp edge. You can use one of those bath netting sponges and take that apart and get some java moss and attach that to the rock. Put the netting over that and twist it on the bottom and use a tie wrap to secure it and you have a really nice moss rock. Easy to trim and take out. I'm glad you got that for free as they know they can't sale broken peices.