Georgian Neoclassical hardstone cameo ring of a young Nero

This is a fabulous hardstone cameo ring, dating to the early 19th century. The ring is made from 18k gold (tested but not marked). It is set with an onyx cameo, sculpted with a male head - likely a rendering of a younger Nero. The gem is nicely cut and polished, it could be older than the early 19th century, certainly 18th or even a Roman original? 

OBJECT: Cameo ring

CULTURE: Classical Revival

DATE: early 19th century or older

MATERIAL:  Onyx & 18k Gold

SIZE: 23mm x 22mm x 2mm ( cameo 15mm x 11mm x 4mm)

RING SIZE:    8 1/2   (US)      Q 3/4  (UK)

WEIGHT: 3 grams

PROVENANCE: Formerly in a private collection, Cambridge.