An unusual diamond, ruby and enamel ring, second half 19th century

An unusual diamond, ruby and enamel ring, second half 19th century, the wide gold band with a hammered finish, centring on a snake biting its own tail, set with cushion-shaped diamonds, its eyes set with cabochon rubies, the interior with a spray of forget-me-nots in blue and green enamel, and the initials JR with a knot in deep blue enamel, size S, stamped 18, maker's mark JW