Similar to a diamond the 4 C's to look out for when selling or buying an emerald are color, clarity, carat, and cut.
Unlike a diamond, the most important factor to consider when looking at an emerald is it's color. There are two factors to consider here. Tone, which is the hue of color itself and typically the darker the better. And saturation which is what gives the emerald it's intensity and strength, the more saturation the better.
When talking about the carat of an emerald, you're most likely talking about its size and weight.
A carat converted into the metric system equals roughly 0.02 grams. However, unlike a diamond, some experts will say it's better to buy a small emerald of better color quality than it is to buy a large emerald with poor color quality.
The clarity basically refers to how clear the emerald is. The quality is determined by the specific characteristics found inside of the emerald, such as scratches, shapes, traces and other things. Just about all emeralds have have these, and unlike a diamond; the lack of them should raise your suspicion of whether or not it's real.
The cut is the shape of the emerald and should typically be cut symmetrically with uniform facets to achieve the best color and brilliance.