Gem-quality emeralds are extremely scarce. The geological conditions that allow for the formation of the beryl gem family (beryllium aluminum silicate) are uncommon, and to have chromium present in order to give the emerald crystal its green color is extremely rare. For cut emeralds to be attractive they must have both an acceptably intense green color, and at the same time sufficient transparency to give them brilliance, ie. sparkle, when they are faceted.
Because there is so little of the natural emerald material that reaches this gemmy standard, it is still the case that there will be at least some imperfections, ie. inclusions, in almost all cut emeralds. This said, the inclusions are not a virtue, and the fact that you can see imperfections in your emeralds does not guarantee that you are getting natural material.
The first step in emerald quality, then, is clarity. While not perfect, for the stone to be valuable it must have good clarity, sufficient at least to have sparkle and light in it. Stones that contain sufficient inclusions as to be mossy, or translucent, are of markedly less value.
Next is the color. The more green the stone, with less of a blue or yellow component, and the more saturated and intense the color, the better. It can be too dark, however. You want to be able to see the color when the stone is mounted, so a medium-dark green color is optimal.
Size will also be a factor in the price per carat. (A carat is one fifth of a gram, the standard unit of weight for faceted stones.) The occurrence of emerald crystals with the clarity plus intensity and evenness of color to be cut into gems of more than two or three carats is very rare. Ten-carat faceted emeralds and larger in fine qualities are extremely unusualÂ¸ and tiny differences in the intensity of color and the clarity of these stones make for huge differences in price.
Finally, the cut or faceting of the stone, while important, is less critical than it is in the case of diamonds. Because emerald is so scarce, every crystal that is mined is cut to yield the best stone that it can produce. To cut to standard sizes and shapes would waste too much material. So every stone is unique. The emerald that you like and that fits your budget is yours alone. Your preferences and pleasure are the final determinant, beyond the expert's advice about quality and value.