In nature, emeralds are found as hexagonal beryl crystals which have a deep green color. An emerald, which is part of a lode or which has been worn down, may no longer appear hexagonal. Dark carbon impurities inside the crystal can also give the emerald a 6-pointed star pattern. These types of emeralds are known as trapiche emeralds, and are extremely rare outside the mines of Columbia.
The green can range from yellow-green to blue-green, but it must be a strong green. Although beryls with a light green color have the same chemical composition as emeralds, they are considered to be green beryl instead of emerald. The value of green beryl is less than that of an emerald of comparative quality. A low-quality emerald can be nearly green-black in color.
However, other gemstones also appear in nature as hexagonal crystals which are sometimes green. In particular, green tourmalines are often confused with emeralds. Thus, visual examination alone is not enough to identify an emerald.
The green color in emeralds is caused by chromium. Some beryls also contain vanadium, and some contain no chromium at all. Iron can also make a beryl green.
Both the Gemological Institute of America (GIA) and the International Gemological Institute of Europe (IGIEU) agree that a beryl which has been colored deep green by chromium can be called an emerald. They also both agree that a beryl which has been colored green by iron is not an emerald. However, they disagree over beryls which have been colored green by vanadium.
“Vanadium emeralds,” which contain no chromium, are only accepted as emeralds in the United States. Most North American emerald mines produce vanadium stones.
Outside the United States, most gemological institutes do not consider “vanadium emeralds” to be emeralds at all. Instead, they are usually called vanadium beryl. Thus, when considering worldwide emerald value, the basic rule of thumb is no chromium, no emerald.
A vanadium emerald can be field-distinguished from a standard chromium emerald by examining it under the Chelsea Color Filter. Chromium content turns a chromium-containing emerald red when examined through the filter. Vanadium stones won’t change color when viewed through a Chelsea Color Filter.
The absorption spectrum is the best way to identify an emerald for certain. Emeralds also have a known density range, but other stones overlap the same range. Crystals which are clear enough can be tested for refractive index. Other lab tests are also available, but they won’t work well on rough stones.
