Lancelot’s Ring
The fairy who raised Lancelot – Chrétien does not further identify her – gave him a ring whose stone had the power to free him from any spell or magical illusion. It protected him from harm and to conceal his identity.
Once, for instance, finding himself and his companions trapped in a strong castle in the marches of Gore, and suspecting enchantment, he held the ring up before his eyes, gazed at the stone, and called on God and his lady the fairy, trusting her to hear and rescue him; by the fact that nothing happened, he knew that the castle was real and solid, that they had to fight their way out by natural means.
Later in the same adventure, he used his ring again to prove that the two lions or leopards apparently set to guard the farther end of the Sword Bridge were, unlike the bridge itself and the castle described above, more illusory enchantments.
Vulgate III identifies the fairy as the (French) Lady of the Lake, and tells us that the ring was a last gift she gave Lancelot before leaving him at Arthur’s court to be knighted.