I have never worked with opals before, but I regularly use Zapon
Lacquer (the best is by Zweihorn/ICI if you can get it) to seal
patina on copper. It looks and acts like nail varnish, but is much
stronger. The other possibility is to cast the piece in clear
polyester resin. You may found that anything you put on the opal
diminishes the shine. Why do you have to cover it at all?