Sometime pre-1995, one of the books I read involved the making of glass beads that would have magical body-control properties if they were made to EXACTLY match the iris of the creature to be controlled. The protagonist was coerced into making a certain bead, possibly to control a magical giant (crab? scorpion? lobster?)

The cover may have been beach near a headland with a human character and a the giant creature-with-exoskeleton menacing the human. Or maybe not.

I've connected it a little in my brain with Ties of Blood and Silver by Joel Rosenberg, but based on the list of books he's published, it's prolly just because I read them around the same time and they both involved adolescents making things under duress.