(σμάραγδος)
The emerald is a mineral of the same species as the beryl. It owes its value as a gem to its extremely beautiful velvety green colour, which is ascribed to the chromium it contains. The primary form of its crystal is a hexagonal prism variously modified. It is electric by friction, and frequently transparent, but sometimes only translucent. Flinders Petrie (Hasting's Dictionary of the Bible (5 vols) iv. 620) suggests that the σμάραγδος with which the rainbow (ἶρις) round about the throne is compared (Revelation 4:3) was rock-crystal, as only a colourless stone could throw prismatic colours. But the nimbus or halo may have been emerald in colour and only like a rainbow in form. The fourth foundation of the wall of the New Jerusalem is emerald (Revelation 21:19).
James Strahan.