Thank you for these photos Annie. They show a lot of detail.
Your first photo shows that there is some metal work behind the pendant I was referring to; however, it is not the type of backing I would expect to see to stablise large stones with diamond surrounds in a pendant.
What I can see in your first photo is intriguing, but it makes me even more convinced that this jewel was initially a brooch, as such fine metal work would stablise a brooch with this type of diamond surround, but not a pendant where the movement of the wearer's body has to be accounted for and minimised.
I will have to try to find photos of the type of metal backing I am referring to.
Again, many thanks for these photos. I have enjoyed seeing them.