History and Formation
An emerald is a precious version of a beryl gemstone, green because of trace amounts of chromium. In the United States, a green beryl with traces of vanadium instead may also be called an emerald. Emeralds as gems go back at least as far as Egyptian antiquity (1500BC), traded by the pharaohs into Asia. Later, the Romans obtained emeralds from Celts in what is now Austria.
Emerald commerce changed completely when Spanish conquistadors discovered the Chivor Mines in Colombia in 1545 and then the Muzo Mines. Trade in volume from the New World built rapidly and has continued. Most (70-90%) of the world's emeralds now come from Colombia.
Other more recent sources of emeralds are found in Zambia (the second-largest source), Afghanistan, Brazil, the United States, and elsewhere.
Mining and Production
Colombian mines are tunnels up to 400m long. Gem-bearing material is drilled from the surrounding rock, then carefully crushed and sorted. Final sorting is done by hand, after which the stones are examined and graded. Zambia uses open-pit mining.
Properties and Uses
Emeralds are a variety of the gemstone beryl with a color ranging from yellow-green to deep green caused by small amounts of chromium. Beryl is hardness 8 on the Moh scale, not as hard as ruby or diamond. Emeralds always have flaws, called inclusions, which make them more fragile than other precious stones.
A raw gemstone must be evaluated and cut to maximize its value. Inclusions reduce the clarity of a gem. Therefore, emeralds unlike other precious gems may be given the smooth oval shape of an opaque gemstone. The clearer stones are given facets very carefully.
After shaping an emerald, it is common to treat the surface to reduce the visible effect of the inclusions. Typically, an emerald is coated with a clear oil of the same optical properties. A person buying an emerald should always learn what treatment was given. Thereafter, the owner should not expose the stone to chemicals or extreme temperatures.
Emeralds can be grown synthetically and sold as such. They are chemically identical to natural emeralds, though they fetch a much lower price.