What an interesting photo Dave! Initially, I wondered if you had discovered an unknown brooch or indeed a photo showing when the pendant to the sapphire necklace was worn as a brooch. But, then I realised it was light bouncing off the diamonds surrounding the central stone which was creating a different impression.
You were correct in thinking that perhaps the pendant was obscured by Her Majesty's dress. In other photos from the same occasion the pendant can be discerned more clearly. For example as in the photo below.
In some of the other photos both the shape of the central stone and the large diamonds surrounding it can be seen more clearly, so I think that HM was wearing the Albert sapphire brooch. What do others think?