I'm finding the Getty Images photos below confusing for a number of different reasons. I've not seen the brooch before and I can't think of any description of a jewel that it fits. Perhaps it is an Australian gift I'm not aware of? Could the central bit be an opal, but I can't even guess what the outer section might be made of!

I can't find any photos on other photo websites as it would be great to have a coloured one! The photos are dated 11th October 1981 leaving St. Peter's Cathedral in Adelaide. They seem to be photos of other photos so perhaps the date isn't accurate, although the 11th Oct 1981 was the last full day in Australia before the Queen left for New Zealand.

Does anyone know anything or has any suggestions about these photos, this event, or the brooch?