This gem represents the head of the god Zeus Ammon--a popular deity who embodied aspects of the chief god of the Greeks, Zeus, and of the Egyptian deity, Amun, whose sacred animal was the ram. Depictions of Zeus Ammon combine the bearded head of Zeus with the ram's horns of Amun.
Zeus Ammon's principal sanctuary was in the Siwa desert in Egypt, where his oracle was located. Following Alexander the Great's visit to Siwa, he claimed to be the son of the god, and, thereafter, he was depicted on his coinage wearing the horns of Zeus Ammon--a motif that Alexander's successors applied to their own coin portraits.
The ornate gold ring into which this gem is set is modern.