I wonder if there might have been a mishap with the brooch, HRH appears to be holding it as she entered the event but was not wearing it on the way out. She appears to have kept the coat on while indoors.
I also wonder if the upper of the two pendants might be a sapphire. At first I thought it was the angle and lighting, but the upper pendant is, to me, clearly darker in any photo in which both can be clearly seen.