Looks to be of Greek origin.
The bulging eyes would be the power of Medusa. All who gaze at Medusa would be forever turned into stone. The background "M" as you say would be for Medusa. Perseus conquest of Medusa. He cut off her head, and the blood that dropped from the wound produced the innumerable serpents that infest Africa. That could explain the lack of serpents on the ring. The head on the ring could represent the severed head of the Medusa mounted on Minerva's shield. In Milton's Paradise Lost. Medusa is guarding the doorway to the world of the dead, she prevents the living from entering.
This story seem to fit the ring. If anyone has anything to add or has another idea on what this ring is, I would love to read about it.
Thanks all! (creskol) (AARC)