I usually use a blob of epoxy and try to mold it into a crack or hole in the rock. stick the frag in and let it harden. sometimes this is enough, but sometimes the epoxy just isn't stuck to the rock. I have yet to have a frag that isn't stuck in the epoxy by the time it's hard. Anyway the ones that don't stick, I pull the frag out and put superglue on the bottom of the epoxy (that's now perfectly molded to the shape of my rocks).
When using Super glue remember that the catalyst is water, so it will immediately skin over when you immerse it in water. Your blobs have to be large enough so that when you squish it against a rock it will either form into the pores of the rock, or break the skin on the glue, preferably both.
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