Vintage Black Cameo Gold-Filled Screwback Earrings

I'm not sure what type of stone this is. Could be onyx, jet or even glass. They depict the profile of a woman whose hairstyle and clothing are reminiscent of the Greek or Roman times. They are set into 1/20 12KT gold-filled settings. They are stamped on the top of the screw 1/20 12KT GF. They measure slightly under 3/4" square. Excellent condition.