Due to their hardness diamonds are notoriously difficult to engrave and executing a portrait would present a considerable challenge. Rudolf II is suggested as the owner of a diamond ring in the Kunsthistoriches Museum, Vienna engraved with the imperial double-headed eagle, and it is possible that the above intaglio portrays the same sitter (Emperor 1576-1612) whose 'strange bent of taste gave him an equal appreciation of the artistic, the rare and the curious' and who also believed in the magical powers of precious stones. Cf. A.Somers Cocks Princely Magnificence, Court Jewels of the Renaissance, 1980 no.83, Eichler Kris, Die Kameen im Kunsthistorischen Museum, 1927 no.316 and Y.Hackenbroch, Renaissance Jewellery, 1979