This is a Victorian Mourning Watch Fob Locket Pendant with layered onyx on one side and carnelian on the other, all set in 10K gold. The locket was sealed when I got it, but I opened it. Inside is a sealed side with something in it! The other has a picture holder with no picture in it. The “glass” appears to be rock crystal and is broken. Dates from the 1880s.
There is an example of this type of locket on page 200 of Collecting Victorian Jewelry by C. Jeanenne Bell (KP Books ©2004). Measures about 1” tall, 1/2” wide and 3/8” thick. Excellent condition. A wonderful addition to any Victorian jewelry or locket collection.