Many many thanks for doing these comparison photos Nellie.
The sapphires belonging to Princess Diana, Princess Anne and Princess Mary are all obviously different.
I am even questioning if the large sapphire and diamond brooch belonging to Princess Mary is the same as the large sapphire element in her tiara. I think it is possible that the diamond surround is built into the tiara and that the diamond centre can be removed and replaced by an unadorned large sapphire - but I am by no means sure of that.