do you know what metal the ring band is made from? Is it gold, silver, gold plated? If it is gold, do you know the caratage? Is there a gold hallmark? If the ring is handmade, there may not be a gold hallmark. The metal content will give you a clue about the cameo quality. It would be more appropriate, for example, to have a genuine shell cameo mounted into an 18 carat solid gold ring.
There is another way I have discovered Angele to tell if the cameo is plastic or shell. It's the famous teeth test! The test is also used to tell a cultured pearl from a Pearl Imitation.
Touch your lower front teeth to the cameo. Plastic has a slightly warm feeling than shell. Composite Cameos of plastic warm up when you touch them. Shell on the other hand is cool and feels much harder like a porcelain teacup. It's all about conductivity.
Concerning the subject of the Cameo, it appears to be a classical subject, with Greco-Roman straight nose etc. However, I would like to have my research team find out more information about the subject on your cameo ring. Please Stay Tuned.