Actually, neither Menkes nor Field claim that the Sri Lankan sapphire in a double row of diamonds was in QEQM's possession before she gave it as a wedding gift to Princess Diana. It appears that everyone just took it for granted that it must have been an heirloom, while it possibly isn't.
Another rare photo of Diana wearing the piece as a brooch, and the brooch before/after its (most susccessful, IMO) transformation by Princess Diana: