Emerald is the green to greenish blue variety of beryl, a mineral species that also includes aquamarine as well as beryls in other colors.
Gem experts differ on the degree of green that makes one stone an emerald and another stone a less-expensive green beryl. Some people in the trade tend to give the name emerald to any green beryl colored by chromium. But to most gemologists, gemological laboratories, and colored stone dealers, it is more correct to call a stone green beryl when its color is "too light" for it to be classified as emerald. Even among that group, however, there's a difference of opinion about what's considered "too light."
GIA uses lab-graded comparison stones to determine if the green color is dark enough and saturated enough to be called emerald.