Does any one know anything about the Thynne family tiara?

If the lack of photos in the newspapers is any indication, the tiara remained in the vaults for many years. Yet, it surfaced a few years ago for the wedding of Viscount Weymouth, heir to the current Marquis of Bath, head of the Thynne family.



Initially, I assumed that a necklace had been used to enlarge the tiara but then I discovered these photos,

1938




which makes me wonder if the lower segment is not a permanent part of the tiara. But I suspect that the lower section was not part of the original design.

Any ideas about which gemstone is used in the lower elements?