Emeralds are a type of hexagonal beryl crystal. Pure beryl, or beryllium aluminum cyclosilicate, is always colorless. Most beryls found in nature are not pure. The type of impurity determines the color of the beryl crystal.
In emeralds, the impurity is caused by chromium. Unlike several other types of impurities in beryls, chromium can only cause a green color.
An emerald’s green color 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 classified as green beryl instead of emerald. The value of green beryl is considerably less than that of an emerald of comparable quality.
Some beryls also contain vanadium as well as chromium, while some beryls contain vanadium instead of chromium. 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 is an emerald. However, they disagree over beryls which have been colored green exclusively by vanadium.
Vanadium emeralds, which contain no chromium, are only accepted as emeralds in the United States. Most North American emerald mines produce vanadium emeralds. These gemstones tend to be grassy green in color.
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 emeralds won’t change color when viewed through a Chelsea Color Filter.
The absorption spectrum is the best way to identify an emerald for certain. Chromium in beryl has known absorption lines, but vanadium has no narrow absorption lines. In fact, a vanadium emerald may broadly absorb the entire red spectrum, which removes any chance of it turning red or pink under a Chelsea Color Filter.
A few emeralds also contain dark carbon impurities inside the crystal, which can be cut to 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.
Iron impurities can also produce a greenish-yellow color. True emeralds can contain iron impurities in addition to chromium and vanadium. However, beryls which only contain iron impurities are not emeralds, but heliodors.
