Bague au firmament
A very neat example of a firmament ring, circa 1800. The gold trifurcated shank supports a diamond-shaped head set with a plaque of royal blue enamel surmounted by a large rose-cut diamond of approximately 3/4 of a carat within a surround of rose-cut diamonds, set silver. Such rings, made popular by Marie Antionette, were known as bagues au Firmament [rings of the Heavens], as the celestial blue ground evoked the night sky and the diamonds twinkled like stars. The ring is size L [US 5.5] and the head of the ring is 3/4 of an inch long and 1/2 an inch wide. Immaculate.
Details from a portrait of Marie Antoinette wearing bagues au firmament