A 19TH CENTURY BLUE GLASS AND DIAMOND RING

A 19TH CENTURY BLUE GLASS AND DIAMOND RING, the oval blue glass plaque decorated with a rose-cut diamond within yellow gold filigree mount, maker’s mark and Swedish control mark, stamped 18k, size R

The ring is very similar to a bague au firmament which were popularised in France at the same time, the single diamond in this ring possibly representing the north star