I like a combo of super glue and epoxy putty. put superglue on the frag, then the epoxy, then more superglue and smash it to the rock.
For epoxy I prefer PC Plumbing epoxy. It's gray, and cures in 3 min. You can find it at HD with all the glues and aquamend and stuff. Perfectly safe, it's the same thing as all the other epoxies, just with more hardener
for superglue, the 3 tubes of gel for $1 at the dollar store can't be beat