I think there is a time when a gemstone can be too large to work in jewellery, and the Andamooka Opal is one of those.

Perhaps it should have been fashioned into a paper weight or other decorative display ornament.

Prior to the government of South Australia purchasing the opal there had been a sketch made of a necklace incorporating five opals. Perhaps that might have been more appealing to QEII, although I suspect that opals are not a favourite of HM.