Exceptional Stuart period gold, rock crystal, and enamel commemorative, mourning or Royalist supporters ring c.1690-1710. 
Gold thread cypher for M ,possibly Mary II (1662-1694) daughter of James II.
The enamel work more colourful than is usually seen on these rings, orange crown like chevrons decorated with white enamel dotted with black, in imitation perhaps of ermine trim. Though much enamel is absent from the shoulders you can clearly see the shank was formerly  enamelled in white and colour in a scroll pattern around most of the hoop. A rare ring  possibly with Royal connections.
Size 7 3/4. 2.98grams.
Mary II was widely mourned in Britain. She died December 28th during a cold winter in which the Thames froze. She lay in state, embalmed, in the Banqueting House in Whitehall until she was buried March 5th in Westminster Abbey.