Once the epoxy is completely cured and any unreacted material is completely cleaned off, it's just inert plastic.

Laszlo

You could embed a leaf or small flat rock, or anything of appropriate size to make them even more cool.  I remember doing a similar thing at summer camp when I was a kid as one of the craft projects.  That was before we all knew how nasty the resin was (I'm sure it was polyester resin - it was a long time ago).  Of course, the boys all decided to put in gypsy moth caterpillars that were rampant at the camp that summer.  I'm not sure that was what the counselors had in mind when they came up with the craft idea.  PETA wouldn't have liked it.