Emerald
Archimandrite Nikifor’s Biblical Encyclopedia
(Exod 28:17 and elsewhere) - the name of a green precious stone, the third stone in the first row of the high priest's breastplate of judgment. This stone was ordinarily brought to Palestine from Syria. According to Revelation, it forms one of the foundations of the spiritual Jerusalem (Rev 21:19), where, however, it is called smaragd. The purer an emerald's water, and the more transparent its green color, the greater the stone's value. The emerald's beautiful color, transparency, and brilliant luster have long attracted lovers of beauty to this precious stone.
Pliny says that the green of trees gives great pleasure, but no object can compare with the green of the emerald. Indeed, when the sight is wearied, it is enough to look upon an emerald for the eyes to be refreshed.