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 coloured by chromium. But to most gemmologists, gemmological laboratories and coloured stone dealers, it is more correct to call a stone green beryl when its colour 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 colour is dark enough and saturated enough to be called emerald.