This is a rare ancient Roman magical gem intaglio, dating to the 2nd - 3rd century A.D. The main device is of a square divided into four, each point having a star. Inside we see the heads of Jupiter, Mercury, a scorpion and an eagle or raven. It has been suggested this is a representation of the seasons, but I feel it is more of a magical gem associated with the mystery religions. It is certainly very interesting and important little gemstone.