Sapphire, a dark blue stone.  Protected the holiness of the eyes and body and kept one from arguments and promoted shalom.  Designated for the Tribe of Yissachar because this Tribe was famous for their Torah study, and the Torah enlightens those who study it (Tehillim 19:9).  Because of this Tribes Torah Study, they were worthy to receive the same stone which the Ten Commandments were engraved on.