A diamond and blue glass ring, 'bague au firmament', late 18th century

A diamond and blue glass ring, 'bague au firmament', late 18th century, of cut-cornered rectangular outline, centring on a blue glass panel set with a vertical line of circular-cut diamonds surrounded by smaller cushion-shaped diamonds, within an outer border of cushion-shaped diamonds, to plain shoulders, mounted in silver and gold, size L1/2, front measuring 2.5 x 1.6cm