File:ElizabethI cameo onyx.jpg

A sardonyx cameo of Queen Elizabeth I.

From:

http://www.royalcollection.org.uk/collection/65186/elizabeth-i

Quoting from that page:

"Creation Date: Cameo: c. 1575-85; Mount: early 18th c.
Materials: Sardonyx: brown and white; open gold mount with claw settings, suspension loop and ring"

It goes on to say:

"Extensive studies of the Tudor portrait cameos, in particular those representing Elizabeth I, have identified overwhelming numbers, leading to the assumption of a special form of large organized court workshop. The cameos vary in size, this being one of a group of six with the largest dimensions, all of which are of superb quality. The others are in the Cabinet des Médailles, Paris, the State Hermitage Museum, St Petersburg, the Victoria & Albert Museum, London, the Devonshire collection, Chatsworth House and the Kunsthistorisches Museum, Vienna.

The cameos were principally used as presents to foreign monarchs or loyal servants: Sir Francis Walsingham is depicted wearing a cameo of the Queen of the same type and size in a portrait of c. 1587; Sir Christopher Hatton is similarly shown wearing a cameo of the Queen on his gold chains, c. 1585."

Please see the "royalcollection" link above for further information, and an excellent, high-resolution picture.