Emerald is the green form of beryl. The finest emerald has the colour of green grass with a very faint hint of blue. Flawless emeralds are very rare. Since early Greek times, emeralds have been boiled in oil. This hides some of the flaws, which are actually tiny cracks, because they become filled with oil and more difficult to see.
Emerald was believed to bring faithfulness and to give the ability to predict the future.
Because emeralds are valuable, imitation emeralds are often made by inserting layers of green glass, or a green cement, between pieces of aquamarine or quartz. The resulting stone has almost identical physical and optical properties to emerald.