Good Morning Gillian the mark SILV on the silver metal indicates it's SILVER - Gillian, are you able to tell if it's a "modern" 20th century cameo, vintage cameo, or a highly collectible Antique Cameo? The subject provides dating clues.
Composite cameos which are not genuine can look very close to the real thing and some can even pass the "tooth test". As your ring is open-backed - great, we can examine from reverse.
Hold the cameo to the light, with the reverse of the cameo facing you, natural light is best; genuine shell cameos often appear to have a "grainy" surface texture often with "grain lines" visible, similar to ivory, which is also an organic material.
The back of the cameo material will not be perfectly flat , if a genuine shell cameo. Shell cameo, especially noticeable in the larger cameo beauties, are usually slightly inverted/convexed when looking from the reverse.
In the 1960s there was a short revival in Victorian style with jewelry that took the form of plastic and resin CAMEOS, such as the *Collectable* GERRY'S and JULIANA Cameos.
