18 ct ring with ancient (BC) agate Roman intaglio

I always buy these ancient intaglios when I can find them. There is one London dealer who normally has something with this sort of age, and he is quite knowledgeable. This one features a shank from the 19th century, and the agate intaglio (of some unknown god I think) has been set in a swivel setting. The intaglio dates to no later than the first century, and most likely before the time of Christ. the ring is a size 8 and 3/4, though it could be sized by a jeweler used to working with antique jewelry. It is crafted of unmarked, but tested, 18 ct gold, and it weighs 3.95 grams. Pieces of this age and in this good condition are very rare.