Found something pretty similar http://www.rubylane.com/item/197251-282/Cameo-Brooch-Pendant-Classic-Greco-Roman
I am not sure as to why we would know it.... is it stolen? Cameos are hand carved (traditionally) and so are all unique. Many are the same depiction, but they vary. Nowerdays they are being done by machine, and are even monkeys and animals are are very expensive! Go figure.