Almost every emerald is flawed, so jewellers grade them by naked eye
Diamonds are judged under tenfold magnification, but emeralds are so often riddled with inclusions and cracks that an emerald counts as flawless if nothing shows to the unaided eye. Most are also soaked in oil, frequently cedar oil, to fill surface fissures and make them clearer and sturdier.
Emerald is a green form of the mineral beryl, tinted by traces of chromium, sometimes vanadium, with iron also influencing the exact shade. Beryl rates 7.5 to 8 on the Mohs hardness scale, yet because most stones are full of inclusions their toughness is rated poor. Along with diamond, ruby and sapphire it belongs to the traditional big four precious gems. Its name travelled through Old French and Latin from the Greek smáragdos, green gem, which itself may have Semitic, Sanskrit or Persian roots.
Colour matters most in grading. True emeralds range from yellowish green to bluish green with dark green dominant, and only medium to dark tones qualify; paler stones are simply called green beryl. Oiling after cutting is standard practice, and synthetic oils and polymers with a similar refractive index are also used.
Mining is ancient. Egyptians worked deposits at Mount Smaragdus from around 1500 BC, and the Romans, Byzantines and later Islamic rulers exploited them on an industrial scale until Colombian finds made them obsolete. Colombia has long been the leading producer, supplying 47 percent of world output in the decade before 2005, though its share has slipped. Zambia comes second; its Kafubu River deposits provided a fifth of the world's gem-quality stones in 2004. Zambian emeralds tend to be clearer, while heavily fractured Colombian stones are prized for their intense colour.
Laboratories now grow emeralds too. Carroll Chatham produced the first commercially successful synthetic, probably using a flux method, and Johann Lechleitner of Innsbruck marketed hydrothermal stones in the 1960s under names such as Emerita. Emerald is the traditional May birthstone. Old alchemical lore credited it with neutralising poison and blinding venomous creatures, and the French writer Brantôme claimed Hernán Cortés had a looted Mexican emerald engraved with a Latin verse about John the Baptist.
Source: Emerald
