They are not marbles--they are smiliar though. They are polished, frosted glass "pebbles" that are made for aquariums. Like I said, I am planning on switching to sand when I put them in their bigger tank soon. So far, they have not had any issues with the pebbles--WAY too big for them to even consider eating at this point. They are able to shove them around a tiny bit, but I have been keeping a close eye on them and it doesn't look like they are at risk of getting caught in them....still one more reason for me to switch to sand, though!