The Queen does not appear to have any great partiality for opals; many of the jewels given to her and other members of the royal family have never been seen in public, although this brooch has appeared on a few occasions.
I have often wondered if the Queen wears this because it was a gift from returned servicemen.
At the risk of starting WWIII I will reiterate my opinion that not all the opals are black opals, despite what media reports said at the time the brooch was presented as a wedding present. To me this seems very clear now that we have good views of the brooch.