While to give the best answer with up to date current value, a photo would be very helpful.
Australian opals are much sought after. On my trips there I have collected opal jewelry.
I learned that the more fire they had in them, the more valuable they are.
As a quite large 1" opal surrounded by silver scrolls, your antique opal brooch would have a retail jewelry store value of $400-600 depending on the "fire" in the opal and the location of the retail venue.
Am insurance replacement value would be $550 - 700. A gemologist can tell you the value of the stone but not the antique value of the brooch. Once the stone is appraised, then an you should consult an estate jewelry specialist.
A private seller to a dealer or at auction can expect 30-60% of retail value.
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