I haven't as yet seen a cameo quite like your one! I believe this cameo was was used to clip onto a dress or coat. The finding on the reverse is the clue here. Dress clips were functional and popular in the 1930s. Woman began to wear dress clips on hats, on the V of a neckline or the square of the neckline.
Usually the "clip" on the reverse have a flat hinged clip with prongs that grasp the fabric ot hold the clip in place. The clip on the back of our cameo is quite different.
Dress clips were very popular in the 30s and 40s. They were functional and this was the appealing feature - they could also be slipped over a cord or chain and worn as a necklace. Dress clips could also be converted into a broach by placing a safety pin inside a shirt or jacket so that the pin bar goes through the material, then you simply drop the clip over the bar and voila- you now had a broach.
I do not think this cameo is shell or coral, it's more likely a resin, synthetic or glass material, and of course it had to be - shell would be much too fragile a material for a dresss clip which is worn on the outside of a dress as opposed to a shell cameo that is worn close to the person.
