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- A brief vision he had of swirling cloud, and in the midst of it towers and battlements, tall as hills, founded upon a mighty mountain-throne above immeasurable pits; great courts and dungeons, eyeless prisons sheer as cliffs, and gaping gates of steel and adamant: and then all passed.6.3 -- Book 6 Chapter 3 -- Mount Doom (95% in)
- It was shaped like the helms of the Guards of the Citadel, save that it was loftier, and it was all white, and the wings at either side were wrought of pearl and silver in the likeness of the wings of a sea-bird, for it was the emblem of kings who came over the Sea; and seven gems of adamant were set in the circlet, and upon its summit was set a single jewel the light of which went up like a flame.6.5 -- Book 6 Chapter 5 -- The Steward and the King (64% in)
There are no more uses of "adamant" in The Return of the King.
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