The Eleventh Foundation Stone of the Heavenly Jerusalem is the Jacinth.
Jacinth indicates souls which are always given up to heavenly contemplation, and which, in a certain way, and so far as is permitted to mortals, approach the angelical life. These are enjoined, “to preserve the heart with all keeping of observation,” lest on the one hand, with too much subtlety of thought, they dare to inquire into things higher and better than themselves. For, “It is the glory of the Lord to conceal a word.” And this is to philosophise with caution concerning God, and the Man Christ; or, lest on the other hand, through the torpor of sloth, they fall back on the weak beginnings of faith, and “the first principles of the words of God.” Yea, rather let them walk in the pathway of the King, and go on, protected by “the armour of righteousness on the right hand and on the left,” and by a fit observance of the time let them change their face with the sky, and say to the beholders, “For whether we be beside ourselves, it is to God; or whether we be sober, it is for you.” As a jacinth surrounded by a cloud, he speaks thus, “Ye are not straitened in us, but in your own bowels;” and again, “For I determined not to know anything among you, save Jesus Christ, and Him crucified.” And as one seen in a serene light, he says, “But we speak wisdom among those who are perfect.”
[The Explanation Of The Apocalypse, by the St. Bede the Venerable Bede, 7th Century Doctor Universalis]