For emeralds the most important factor when it comes to value is color. Apart from this there are several other factors. Those factors are carat, inclusions, transparancy, cut, signs of treatment(s) and color saturation.

Because the gemstone emerald is part of a larger group (beryl) it might occure that a different color is more prominent then green thus affecting the hue of the gemstone. In this case you no longer speak of an emerald which decreases it's value. 

Most emeralds contain inclusions that can be seen with the naked eye. Their presence is commonly accepted since emeralds without visible inclusions are a rarity. Some inclusions are location specific and tell from what country an emerald originates. One specific form of inclusion is called trapiche. Inside the emerald inclusions form a star-like pattern. Trapiche emeralds are very rare and valuable.

The transparancy of emeralds is often improved with a clarity-enhancing treatment. This is achieved through the use of an oil. These treatments are very common and most emeralds have had such a treatment. The severity of such a treatment makes a big difference in what value an emerald has.

Beryl is found in certain varieties known as emerald, aquamarine, bixbite, goldberyl, goshenite, heliodore, maxixe, morganite, and pezzotaite. Every variety has it's own color and value, because some colors are very rare while others aren't.