Pretty much what is ask is what you need to do. The diamond should
be in a thicker plate, probably, like 1mm or something. You can't use
a screw, because there's going to be a big hole drilled in the middle
for the stone. So, you need to drill it and make the plate ready for
the diamond, and then solder a piece of tubing that's big enough to
go around the hole, and make sure it's not going to get cut away
when opening up for the pavilion of the stone - happens sometimes.
Then have the onyx opened up for the tubing - not vice-versa. Have
them add a little notch on the side of the hole, which will keep the
top from spinning. You can use most anything for the tubing,
traditionally people even use silver. The onyx is quite fragile, and
you're gonna put the squeeze on it....