Emerald (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 (HDB [Note: DB Hastings’ Dict. of the Bible (5 vols.).] iv. 620) suggests that the σμάραγδος with which the rainbow (ἶ ρις) 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.