Arabella or Elizabeth Stewart
From lisby1's photostream on flickr. See a color version of the same portrait identified as Elizabeth Stuart here. According to Yale University, this could be "One of three miniatures in watercolour on vellum, set inside an ebony veneered 17th or early 18th century case with inner gilt slips."
Young Arabella Stuart poses in Elizabethan dress with grotesquely puffed sleeves and a wheel farthingale. Her hair is arranged in what appears to be two very long curly falls.
Keywords: Stuart family, English, long curly coiffure, hair jewelry, square neckline, modesty piece-bertha, long puffed sleeves, cuffs, girdle, vee waistline, wheel farthingale, necklace, draped necklace, earrings, jeweled sleeves, bracelets, jeweled skirt