Double whammy: it's a mourning AND a memento mori ring. It was dedicated in 1779 for a twenty year old woman named Mary Sewell who was unmarried. The white enamel indicates "virginity" but really, we can only speculate about that. Unmarried is a surer bet. On the reverse side, it says "Be ye also ready", meaning that death can come at any time, so behave thyself. The anchor = hope, the urn = mourning, the weeping willow = grief.
Mary Sewell OB 12 July 1779 AE: 20. Underside of the head engraved "Be ye also ready"
MATERIALS: 14k gold (tests), white enamel, rock crystal over ivory sepia and hair
AGE: dedicated in 1779
CONDITION: Very good - minor surface wear commensurate with age and use
SIZE: 6.75, can be slightly resized; 7/8" x 5/8" head, 1/8" shank