The Four “C”s of Emeralds
Similar to diamonds, you must consider the four “C”s when evaluating an emerald. The four “C”s are: Color, Clarity, Cut and Carat Weight. Although, these are the same four “C”s, their order of importance is completely different. With diamonds, you are primarily concerned with clarity and with emeralds you are primarily concerned with color.
Color: Color is the most important aspect when considering emeralds. Generally speaking the darker the color the more valuable the emerald. However, just because an emerald is dark green does not make it valuable. A fine emerald will have a dark blue green color while at the same time be translucent and brilliant. There are many dark opaque stones that are very cheap.
Clarity: Emeralds are one of the most included precious stones. Natural Inclusions in emeralds are sometimes called 'gardens' (because of the green color) and the inclusions in emerald are what cause the wonderful emerald green color. Since emeralds are so highly included, a high percentage of emerald rough (normally 80 to 95%) must be cut away to create a gemstone. Thus, the high weight loss is one of the primary reasons they are so expensive. This is also one of the reasons that even synthetic emerald that is grown in lab is expensive because it too has inclusions. There is no set scale like diamonds that describes the clarity (F,VVS,VS,etc). Although many jewelers and organizations have adapted this scale.
Cut: The faceting, shape, width and depth of the emerald make up the cut. The ideal cut emerald will be symmetrical and have uniform facets that provide for maximum color and brilliance. If the cut is too shallow, light will be lost at the bottom of the stone and the emerald will not have maximum brilliance. If the cut is too deep, the light will escape out of the sides and the emerald will appear dark. Remember, because emeralds are expensive and sold by weight, the cutter is pre-disposed to try to save as much weight as possible. Also, the shape of the rough will most likely pre-determine the final shape of the cut emerald. Round emeralds are least common because you must waste more material to cut a round and the rectangular step cut known as the "emerald" cut is generally thought to compliment the end result for emeralds.
Carat Weight: The actual carat weight is the least determinant factor in the value of an emerald. Obviously, just because an emerald is large does not mean it is expensive and vice-versa. A large 8 carat dark brilliant stone would obviously be more expensive than a 2 carat stone with the same qualities. Also, 1 carat stones are certainly more desirable than stones less than a carat in weight, however it is not as an important a factor as it is when considering diamonds. Thus, the color, clarity, and cut are certainly to be taken into consideration before carat weight.
Generally speaking, most colored stones are evaluated using the above criteria as a base guide. If you are looking for a good book that describes how evaluate and buy colored stones, we highly recommend Colored Gemstones, 2nd Edition: The Antoinette Matlins Buying Guide by Antoinette Matlins. It is an excellent general guide for buying and caring for emeralds, rubies and sapphires and other colored gemstones. There is also some good information on pricing, gem certificates and determining value.