Mary, Duchess of Devonshire wore the jewel as a necklace and tiara, as Ashdean mentioned, and also wore one of the motifs as a brooch. I don't have the book, but someone once posted here that it is not mentioned in the chapter on the Devonshires in Diana Scarisbrick's Ancestral Jewels. Considering that and the fact that it has not been seen on any previous or subsequent duchess makes me think that it could have been Mary's personal property and was perhaps left to one of her daughters.