A fine quality bague au firmament ring, circa 1780. Two innovations in French jewellery design in the latter part of the 18th century were firmament and enfantement rings, designed respectively to celebrate the good news that Louis XVI and Marie Antoinette were expecting an heir and then to celebrate the birth of the Dauphin.  On this bague au firmament the high carat gold  shank supports a plaque of vibrant foiled cobalt-blue glass set with a central rose-cut diamond, within a surround of rose-cut diamonds, all diamonds set silver. The celestial blue ground evoked the night sky and the diamonds twinkled like stars. The ring is size L [US 5 and 1/2] and the head of the ring measures 1 and 1/4 inches by 3/4 of an inch. Immaculate.