Marble for sure is not safe, it can leach many toxins. For one, most marbles carry some form or another of Arsenic, a known lethal toxin. I doubt the quantities would ever build up in an aquarium, especially one with regular water changes. Most ceramics are fine, however.
Overall, it's much safer to use Travertine tiles. Tavertine is a limestone product. You will want to find the whitest ones you can. The darker they are, the more impurities they have in them. I bought a 12" x 12" section with 144 1" tiles for $8. They are glued to a mesh, so the easiest way is to pull them from the mesh and then soak them in a bowl of vinegar overnight. Once they soak, and residual glue is easily peeled/rubbed off. They also have larger sizes for growing out Chalices, Favias and the like.
Also, the hexagon style travertine products make really good looking frag discs. :)