I think that I would solder a post on the back of the gold & diamond
piece, and drill a hole in the center of the onyx tablet. Insert the
post through the onyx to the backside,then cut the back of the post
in 1/2 or 1/4 if poss, and split it open to hold the gold in place
in addition to some 2 part epoxy.
My first time writing back to this WONDERFUL site, I hope I did it
right. ;> Laura Guptill Jewelry