A George III diamond and blue enamel ring, the navette-shaped closed-back section with blue enamel

A George III diamond and blue enamel ring, the navette-shaped closed-back section with blue enamel ground, applied with a row of three graduated cushion-shaped diamond clusters, with cruciform diamond terminals (two diamonds missing), with further outer border of cushion-shaped diamonds in silver on gold, with later inscription to the reverse 'Diamond Jubilee 1837 - 1897 GB to CLB, size N