This miniature is in fact a photograph (probably an albumen print) painted over in gouache. The over-painting has been attributed to Elizabeth Siddall's husband, Dante Gabriel Rossetti, the English poet and founding member of the Pre-Raphaelite Brotherhood, formed in London in 1848. The lavish frame in gold, opal, sapphires and diamonds (made in London) was added in 1906 by J. Pierpont Morgan, a previous owner, and it was cataloged by G.C. Williamson in the same year as "Mrs. Rossetti."
The story attached to the photograph is that, after Siddall's death in February 1862, Rossetti gave it to a nurse who had attended at the birth of Siddall's still-born child, and that it passed to the nurse's daughter, who in difficult financial times sold it to a clergyman (see G.C. Williamson, "Catalogue of the Collection of Miniatures, the Property of J. Pierpont Morgan," vol. 2, London: Chiswick Press, 1906, p. 116). This story is also told by Williamson with some variations in "Mr. J. Pierpont Morgan's Pictures, the English Miniatures, V.," Connoisseur, 8:70, June 1907, pp. 71-76, p. 75; "Stories of an Expert," 1935, pp. 36-39; and "The Cases of an Art Expert, II, The Rossetti Miniature," Country Life, 80, 11 July 1936, p. 35-6.).
The costume is correct for ca. 1860 and the portrait shows strong resemblances to Siddall as depicted in several studies and paintings by Rossetti. The pose recalls the posthumously completed painting "Beata Beatrix" (ca.1864-70), Tate Britain, London, N01279.