In the black abyss there appeared a single Eye that slowly grew, until it filled nearly all the Mirror. So terrible was it that Frodo stood rooted, unable to cry out or to withdraw his gaze. The Eye was rimmed with fire, but was itself glazed, yellow as a cat's, watchful and intent, and the black slit of its pupil opened on a pit, a window into nothing.
--The Fellowship of the Ring, book II, chapter 7: The Mirror of Galadriel
The brief glow fell upon a huge sitting figure, still and solemn as the great stone kings of Argonath. The years had gnawed it, and violent hands had maimed it. Its head was gone, and in its place was set in mockery a round rough-hewn stone, rudely painted by savage hands in the likeness of a grinning face with one large red eye in the midst of its forehead.
--The Two Towers, book IV, chapter 7: Journey to the Crossroads