The keystone of the arch crashed almost on their heels, and the wall above crumbled, and fell in ruin.
At last they came out of shadow to the seventh gate, and the warm sun that shone down beyond the river, as Frodo walked in the glades of Ithilien, glowed here on the smooth walls and rooted pillars, and the great arch with keystone carven in the likeness of a crowned and kingly head.
There are no more uses of "keystone" in the book.
Show samples from other sources
the keystone of campaign reform was the ban on soft money
Friendship with Canada is the keystone in the arch of America’s foreign relations.