Hi, Depending upon the kind of pattern you are aiming for it could
be, should be, might be - possible to hammer set the gold (22k or
more for softness) into carefully carved, smooth sided, slightly
undercut, grooves and even perhaps into small areas in the stone.
I say this because I've just done some hammer settings of small
transparent Mexican opals into carved agate using fine silver as the
"surround" for the opal and with the whole assembly set in a dish of
cold dop wax.(I think a picture might be helpful here; email me
offlist if you'd like me to mail you a jpeg of the result).
The logic is that if hammer setting can be done with something as
fragile as a transparent Mexican opal, it should work in onyx which
is typically black-dyed agate, hence tough, and much less inclined
to chip than opal.
I might add that I tried the same thing with obsidian. No go. In
each case the opal survived but the obsidian split.
normally Moncton Canada
presently and appreciatively in snowfree Sunland California