Far off the shadows of Sauron hung; but torn by some gust of wind out of the world, or else moved by some great disquiet within, the mantling clouds swirled, and for a moment drew aside; and then he saw, rising black, blacker and darker than the vast shades amid which it stood, the cruel pinnacles and iron crown of the topmost tower of Barad-dûr. One moment only it stared out, but as from some great window immeasurably high there stabbed northward a flame of red, the flicker of a piercing Eye.

Not sure how that wouldn't translate to literally looking like an eye of flame but okay.