Wymanda I think you are correct. I know that the press reports in 1947 stated they were black opals and obviously some are, but!
A review of what has been said previously by posters led me to GillW's post. http://members2.boardhost.com/royal-jewels/msg/archive/1395437799.html The information given to her says the stones represent the various areas where opals are mined, which would account for why one looks as if it came from Coober Pedy and one from Andamooka (I am basing that comment on the characteristic colour identified with those areas.
Re the Andamooka opal. I suspect it is too large for use in a brooch. If the Queen liked opals sufficiently, I suspect she would have had the necklace redesigned years ago. Nevertheless, the sparkle of a diamond border would enhance the stone.
--Previous Message-- : I'd love to see Her Majesty have this Opal : reset into a brooch in a similar setting to : the Albert Sapphire. :