This cameo looks like it might be from the mid-late 1800's. Moulded pieces made from man made material was used widely to mimic onyx or black enamel. In Antique Jewelry Investor's Jewelry Resources you can view a Video about Victorian Mourning Jewelry & Customs.
If the Cameo is made from Onyx which is a hard-stone, it will be worth more. One simple test, with cameo surface against the cheek, does it feel as cold as stone?
