Beth, I do think both your photos show the same ruby brooch. Apart from the fact that coloured gemstones will appear greyish or black in a b/w photo, Princess Marina's veil is adding to the optical illusion: the lozenge pattern of the white meshwork against the background of the dark stones creates an effect that resembles the lustre of grey or black pearls.
I agree that an exchangeable diamond cluster centre of a tiara very likely would have had an additional brooch fitting. And I find it a bit difficult to imagine this rather long ruby brooch as the centre of a tiara design which seems just too straight to adjust to a forehead.