Various articles, principally brooches, pendants, bracelets, and finger rings, decorated with a portrait, usually executed by carving as a cameo or by enamelling as a medallion on porcelain, bone china or copper, or painting on vellum set under a crystal. Rings made in the Roman period often bore portraits, as did enamelled pendants made in the 16th/ 17th centuries and enamelled rings, bracelet clasps, etc., made in the 18th century.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson