Also known as Beli Oculus or the Eye of Belus, this mystical stone was said to brings the person carrying it the power of invisibility. It has been described as a white stone with a black center.
The stone had quite a presence in the Late Middle Ages into the Early Renaissance. According to Siegfried Wenzel, the stone appears in the sermons of Robert Holcot, but says it bears the name of the demon Bel and compares it to envy.
This is interesting, as Isidore of Seville, the Archbishop wrote something different about the name. In book XVI of his Etymologiarum sive Originum, he write that Beli Oculus is named in honor of the King of Assyria, Belus.
Neither Holcot nor Isidore of Seville mention invisibility. This attribute appears to have been a rather late addition to the myth.