Good evening, Shell and ivory are two different materials. Both are classified as "organic gems" though and both have been used throughout the history of cameo making.
Ivory has many imitations, that include glass, plastic, bone and vegetable ivory. It is possible to identify ivory with a hand lens however. Elephant ivory has a visible pattern under magnification - "decussating engine turning" on the surface of cross sections of the tusk. These grainy lines are called "Schreger lines"in Elephant Ivory and usually form angles greater than 115 degrees. There is a photo of this peculiar pattern on ivory at -
http://www.ivoryheaven.com/id1.html
FYI Teeth and Tusks are made from similar, moderately hard dentine.
Are you now able to tell if the cameo material is ivory or shell?
