Emerald is a variety of the mineral beryl. It is colored green by traces of Chromium and Vanadium. Beryl has a hardness of 7.5–8 Mohs. Most emeralds are very brittle.
Emeralds are graded using four basic parameters – the four Cs : Color, Cut, Clarity and Crystal.
Before the 20th century, jewelers used the term water as in “a gem of the finest water” to express the combination of two qualities, color and crystal.
Normally, in the grading of colored gemstones, color is by far the most important criterion.
However, in the grading of emerald, crystal is considered a close second. Both are necessary conditions. A fine emerald must possess not only a pure verdant green color but also a high degree of transparency to be considered a top gem.
In the 1960s the American jewelry industry changed the definition of ’emerald’ to include the green vanadium-bearing beryl as emerald.
As a result, vanadium emeralds purchased as emeralds in the United States are not recognized as such in the UK and Europe.
In America, the distinction between traditional emeralds and the new vanadium kind is often reflected in the use of terms such as ‘Colombian Emerald.’
Emerald is a brittle gem. It tends to chip off easily. It is rarely flawless. Chromium-a hard white metal-causes its rich green color.
In ancient times powdered emerald was believed to cure fever and plague. Colombia is the chief source of finest emeralds.