what a beautiful antique cameo! I think the material may be Sardonyx shell. Note: not the be confused with the striped variety of sardonyx related to the Chalcedony gem family.
The colour of Sardonyx shell is a rich red chocolate/chestnut brown colour and in Italy cameo made from this shell are referred to as Cocoa shell cameo. Sardonyx Shell cameo are the most valuable!
Carnelian Shell Cameo are a different colour than the Sardonyx Shell Cameo and the former shell is generally lighter and more of an orange to pink colour.
A typical Antique Sardonyx Shell Cameo can easily cost as much as $2,500
Cornelian shell cameos are also created in the same way of Sardonxy shell cameos. There is however one major difference in the process that is attributed to Cornelian shell cameos that Sardonxy lack. The amount of layering in the carving of Cornelian shells cameos can include the top outside layer of the shell, mid layer for the profile, and the bottom base layer adding a level of complexity and hue to the overall beauty of the cameo.
The assembly line of artisans carve their own character and expertise into every shell. While one artist may have spent there working life carving noses or hair while another has spent there working life trained to carve flowers. Only a master carver has the experience to carve a cameo in its entirety, meaning the majority of cameos have gone threw an assembly line where multiple carvers add their expertise and flavour to every piece.
This is why Cameo that are signed, like your cameo signed 'Cortese' (which most likely is an Italian a master carver) especially antique cameo are so highly sought after today. It indicates a master carver has carved the cameo in its entirety not an assembly line of carvers. The master art is clearly evident in your cameo Barbara, and the intricate details including the string of pearls on the subject who is quite striking with her profile still crisp and full of energy is reflected in the beautiful chose of frame which also contain pearls.
