Generally, cameo's had open backs and not solid - reason is because it depends on the light to come through to help with the color/highlights.
Like this:
If it was shell....the pink/orange color of a shell is what shows as being the background when the white/off white/pale part of the shell is carved away to reveal a carving aka cameo. Also notice in the picture, a shell doesn't sit flat but is curved.
Cameos were made of stone, lava, bone, etc. The closer look I get at your piece it doesn't resemble shell. Cameos need a hands on for a proper identification. Do you have a local jeweler you can take it to?
