Gem experts differ on the degree of green that makes one stone an emerald and another stone a less-expensive green beryl. Most gemologists, gemological laboratories, and colored stone dealers 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.”

Mineral Beryl
Chemistry Be3Al2Si6O18
Refractive Index 1.577 to 1.583
Birefringence 0.005 to 0.009
Specific Gravity  2.72
Mohs Hardness 7.5 to 8