Emerald or Green Beryl?
If you ask a gemologist, he or she will mostly likely provide a simple definition of an emerald, namely that it is a green variety of beryl.
Is this an adequate definition?
Although an emerald is a green variety of beryl, there also exists a "green beryl" that is less valuable and not as rare as an emerald. Just when does one have a green beryl and not an emerald? After all, they are both beryl and possess a green color.
Some experts simply say that an emerald looks like an emerald and green beryl does not. Other experts rely on the presence of trace elements that impart the green in an emerald, namely chromium or vanadium. Even the experts are not in agreement, are they?
What is a green beryl?
Green beryl is colored by the presence of iron. Iron usually imparts a yellow secondary color in beryl and sometimes even a bluish secondary color. Aquamarine is a variety of beryl and an example of the effect of iron. Heat an aquamarine and the yellow secondary color disappears leaving a desirable blue color. An emerald must have the trace element of either chromium or vanadium to be an emerald, otherwise it is a green beryl. It is a rare event when beryl is formed in a location where it is exposed directly to either chromium or vanadium (or a combination of both). That is why emeralds are so very rare.
What is chromium?
Chromium, by the way, is what makes rubies red. However in beryl, green is the outcome. It is not only chromium that causes green in emeralds but some other trace elements as well (i.e. nickel). In fact, some emeralds have both chromium and vanadium. Most likely the presence of both elements in some degree is necessary to be an emerald. Gemologists and appraisers tend to rely on the gem looking like an emerald and very rarely depend on instruments as a means of separating emerald from green beryl. The instrument needed would be a spectroscope that can display absorption spectrum indicating the presence of vanadium or chromium. In gemological terms, an emerald is a beryl of green color that may have secondary colors of yellow or blue. Its spectra will exhibit absorption lines in the red and blue-violet regions revealing the existence of chromium and/or vanadium. Since most spectroscopes are difficult to use, there is a resistance to using them to define emerald.[1]
Conclusion
Emeralds are green because of the presence of chromium or vanadium. Green beryl is green because of the presence of iron. Emeralds that have a trace of iron will have yellow in them but still have either or both chromium and vanadium.
Footnote: [1] Imagine a rainbow seen through a gemstone. The gemstone blocks certain specific colors from being seen. The gemstone is acting as a filter and we can detect what specific colors are being filtered out when we observe the gems transmitted light with a spectroscope. The reason the filtering action works is because certain elements a gemstone contains act to filter certain wave lengths of color by absorbing them - thus the technical term "absorption spectra." The combination of the colors allowed through the gemstone give us color. In other words, chromium filters out certain colors and those passed through are perceived as green.
