Apologies but I caught up with this discussion only this morning.

I spent a few hours endeavouring to find a description of Queen Mary's jewels for that garden party, but to no avail. There was just the usual reference to her magnificent jewels!!

I wonder if the brooch could be the amethyst brooch given to Queen Mary as a wedding gift by William Burdett Coutts?








The brooch worn by Queen Mary at the garden party looks slightly different depending on the angle of the camera shot -- but





There are other occasions where I think Queen Mary might have been wearing that amethyst brooch, but the photos lack definition to be absolutely sure.