It was not in the Cartier display so I assume that the firm did not purchase the item. The back of the jewel (or the front as it is worn) has beautiful enamel work and a diamond floral motif in the centre. The auction notes indicate that it was created circa 1920
I think that Nellie Melba wore this on a pearl chain and, despite the fact that the images are not 100% clear that it can be seen in the following photos.
What do others think?
Detail from photo above.