I agree with what Franck said in 2015 ---that it belonged initially to Queen Mary and was given to her as a wedding gift by King Edward and Queen Alexandra. It is entirely possible that the central section is detachable and was the jewel QM was wearing in the photo Franck posted last year.
The sketch comes from the Penny Illustrated Paper There are some slight differences but that is to be expected when comparing a sketch to a photo of the actual jewel.