I've used a wet tile saw to cut standard ceramic tile into small pieces. The stuff on the mesh with glue tends to cost a lot more than a standard ceramic tile. Probably don't really need a wet tile saw either, just take a hammer to a tile & rinse off the dust when you're done. Voila, a bunch of irregularly sized, random frag tile pieces. Who says they have to be square? And you can always wrap a blob of epoxy putty with a tail around it to give it purchase on a frag rack.

I've also cut up a few chunks of marble since it's nice & heavy. Works fine & haven't had an issue with any toxicity (spelling?) to the best of my knowledge. If you search the interweb for info on marble constituents, you'll find it's >90% calcite or dolomite. Veins in marble are impurities, generally granite which could contain some nasties, but I doubt that a small chunk used as a frag plug is going to leach enough of anything into your tank to be harmful.