Miniature / pendant?

Miniature or oval pendant, possibly after a portrait of John Erskine, 2nd Earl of Mar (1558-1634), probably painted by Charles Dickson, or Dixon, goldsmith and miniaturist, in a gold frame with agate back, mount engraved on back with 'No. 97', engraving probably 18th century suggesting it formed part of a collection of miniatures at that time: British, c. 1750