Ros, I set quite a bit of boulder opal - it can be quite challenging
and loves nothing better than to chip just as you bend over the last
bit of the bezel, or as you are doing the last bit of burnishing.
The idea you have for setting it sounds OK to me, but, because opal
is rather fragile and doesn’t really take any rough treatment or
being dunked in dishwater too kindly, at least consider something
else - maybe a brooch or pendant.
(OK, I know there are zillions of opal rings out there, some of them
magnificent and valuable stones, but there you go…)