This piece is a bit of a puzzle. The dealer in London that I bought this and some other Scottish jewelry from said it was "a Tassie".
Tassie was a Scottish stonemason, skilled in carving cameos and engraved gems. Sometime in the 1700's Tassie developed a glass formula suitable for casting, from which he made at least 15000 replicas of some of the best European cameos. He also did portrait cameos of contemporary aristocrats and royalty. After he died, his nephew William continued to run the business well into the 19th century. I've since found out, however, that some people refer to any early glass cameo as "a Tassie."
This particular item has no markings of any kind and I frankly think that any attribution to Tassie is open to question, but you've got to admit it is an absolute stunner, whoever made it. Frame is a white metal and is unmarked and untested. Portrait in opaque white glass is applied to a caramel colored glass background. Measures approx. 1 7/8" x 1 1/2" with the tube hinge pin extending nearly 1/4 inch beyond the simple c clasp. Exact age unknown, but probably Victorian. Small ring at the top of the bezel is probably too small for any kind of neck chain, and was most likely the attachment point for a safety chain. No condition problems other than a few light surface scratches on the background.