Gary, try the “Azama Stone-in-Stone” setting. Take a diamond drill
(core or ball may also be used) that is 90-95% the diameter of the
stone you want to set and drill through the onyx. Then take a diamond
ball or cylinder point the same diameter as the stone and from the
back side of the onyx enlarge the hole up to the top, but not
completely through. You want to leave a natural onyx bezel around the
hole (theoretically a 98% opening would prevent the stone from
falling out). Slip the diamond in from the back side, make an onyx
plug (dowel) to hold the diamond up in position. Glue only the onyx
plug to the onyx body. Enjoy. Min in Tokyo.