Very Old Decorative Pendant Glows in sunlight

What you have is a cameo brooch most likely carved from Cornealian shell. Jewelry is impossible to appraise on the Internet. It's best to take it to a jeweler to have the metal tested and the cameo identified (shell? stone? glass?). I'm thinking your brooch as made later because of the clasp. It has a Modern Safety Catch which became popular starting in the 1920s. The cameo could be older though. Here is some more information on cameos: https://www.antiquetrader.com/collectibles/faces-give-clues-to-cameos-age-materials It may be a pendant, too but since I can't see the bale from the photo so I'm calling it a brooch. It was common to make "pendent brooches" and still is! Very versatile!