Franck, the various reports in the Australian press of QEQM wearing a large opal brooch during the tour turned out to be the brooch from the turquoise parure which George V gave her as a wedding gift. (A couple of press reports got it correct and stated it was a turquoise)
For years, I assumed the brooch had an opal, but you pointed out some time ago that it was the turquoise.
QEQM wore it to the Opening of Parliament in Canberra.
A closer view
What has always intrigued me is the brooch HM wore for a visit to Mt Cootha in Brisbane. It has a large central stone. I don't think we have ever identified it.