Of rectangular form, applied with a panel of blue glass and set with rose diamonds. 

Incised number 44095A4. Diamonds are bright and lively, some with small chips and surface scratches. Glass with light surface scratches. Mount with scratches and some discolouration and solder around pinched collets. Clasp closes securely. Overall condition good.
"In response to your inquiry, we are pleased to provide you with a general report of the condition of the property described above. Please note that colour, clarity and weight of gemstones are statements of opinion only and not statements of fact by Sotheby's. We do not guarantee, and are not responsible for any certificate from a gemological laboratory that may accompany the property. We do not guarantee that watches are in working order. Prospective buyers should inspect each lot to satisfy themselves as to condition and must understand that any statement made by Sotheby's is merely a subjective, qualified opinion. Prospective buyers should also refer to refer to any Important Notices regarding this sale, which are printed in the Sale Catalogue, in particular to the Notice regarding the treatment and condition of gemstones.
NOTWITHSTANDING THIS REPORT OR ANY DISCUSSIONS CONCERNING A LOT, ALL LOTS ARE OFFERED AND SOLD "AS IS" IN ACCORDANCE WITH THE CONDITIONS OF BUSINESS PRINTED IN THE SALE CATALOGUE."

The present clasp owes its distinctive style to a deep blue enamel developed in in London 1775 by the enameller Jusen. Applied to a variety of jewels and set with rose diamonds, this style of jewellery quickly found a following both in England and in France, where the rings were known as bagues de firmament due to their similarity to the night sky. The enamel's popularity soon led to imitations, in this case using Bristol glass, a similarly rich blue glass coloured with cobalt oxide also developed in the 1770s around Bristol.

Cf: Ginny Redington Dawes with Olivia Collings, Georgian Jewellery: 1714-1830, Suffolk, 2007, pg. 120, for more information on this technique.