This cameo is the rarer Carnelian-onyx rather than sardonyx, as the stone is more red than brown. This shows up better in some of the photos due to lighting differences. The woman is beautifully carved, and there is a subtle color variation (more beige than white) in her hair and gown. The cameo is in excellent condition, with no damage.
The amazing cameo is surrounded by gold colored plated metal, and there are some areas where that is worn, and the silver colored metal below shows through. The fitting is in the shape of a shield, and decorated in the granulation of the Etruscan Revival style popular in the mid-19th C. It measures about 1-9/16" (4 cm) long.
I believe this piece was originally (about 1870 ) a pendant, and later had a clasp and watch pin added (c. 1900). You can see evidence of this on the back (see photo).