The Fifth Foundation Stone of the Heavenly Jerusalem is Sardonyx.
"Sardonyx. This derives its whiteness from the onyx, and redness from the sard, and it has received from the two the name sardonyx. But there are very many kinds of it: for one preserves a likeness to red earth; another appears to be of two colours, as the blood shines through the human nail; another consists of three colours, the lowest black, the middle white, the highest vermillion. And with this are compared men reddened through suffering of the body, white through purity of spirit, but lowly in their own eyes through humility of mind, protesting with the Apostle, “Though our outward man perish, yet our inward man is ‘renewed day by day;” and again, “I know nothing against myself, yet am I not hereby justified.” And so also the Psalmist says, “Although man walk in the image” of God, that is, from the power of the mind, “yet he will be disquieted in vain,” that is, from the weakness of the flesh. This suffering also is through the weakness of the body, for they “who kill the body are not able to kill the soul;” and humility descends from the frailness of the same body, when it is said, “O wretched man that I am! who shall deliver me from the body of this death?” The sardonyx is rightly placed in the fifth foundation, for it is certain that our body depends on five senses."
[From The Explanation Of The Apocalypse, by the St. Bede the Venerable, 7th Century Doctor Universalis]