Burnham, S.M.: Precious Stones in Nature, Art and Literature (1886): p. 163
Westropp, H.M.: A Manual of Precious Stones and Antique Gems (1874): p. 104
Unknown: 422.16
Comments: Burnham: 'The onyx was frequently engraved, a remarkable specimen being afforded by the Corinthian helmet on jasper-onyx. An imitation of this engraving constituted the chef-d'?uvre of the notorious Poniatowsky collection.'