Beryl is a rare mineral with a chemical composition of Be3Al2(SiO3)6. It is rare because beryllium is an element that occurs in very small amounts in the Earth’s crust. It is unusual for enough beryllium to be present in one location to form minerals. In addition, the conditions in which beryllium is present in significant amounts are different from the conditions where chromium and vanadium, the sources of emerald’s green color, are expected. This is why emerald is rare and only found in a small number of locations.
Today, most emerald production originates in four source countries: Colombia, Zambia, Brazil, Ethiopia, and Zimbabwe. These countries reliably produce commercial amounts of emeralds. Minor amounts of production or irregular production comes from Madagascar, Nigeria, Afghanistan, Pakistan, Canada, Russia, and a few other countries.
Starting in about 2015, significant amounts of emerald with exceptional color and clarity started to be exported from Ethiopia. An editorial on the JCK website speculated that these Ethiopian emeralds might be the greatest gem find in 100 years. An article in Gems & Gemology surveys the production of emerald in Ethiopia through 2017.
Even though the conditions for the formation of emerald are very unlikely, the gem has been found in a diversity of rock types. In Colombia, the country that has supplied most of the world’s emeralds, black organic shale and carbonaceous limestone, both sedimentary rocks, are the ores for many emerald deposits. The shale is thought to be the source of chromium, and the beryllium is thought to have been delivered by ascending fluids.
Many of the world’s emerald deposits have formed in areas of contact metamorphism. A granitic magma can serve as a source of beryllium, and nearby carbonaceous schist or gneiss can serve as a source of chromium or vanadium. These emeralds usually form in schist or gneiss or in the margins of a nearby pegmatite. Mafic and ultramafic rocks can also serve as sources for chromium or vanadium.
Emeralds are rarely mined from alluvial deposits. Emerald is usually a fractured stone that does not have the alluvial durability to persist great distances from its source. Emerald also has a specific gravity of 2.7 to 2.8, which is not significantly different from quartz, feldspar, and other common materials found in stream sediments. It therefore does not concentrate with high-density grains which are segregated in the stream and more easily recovered by placer mining.
