Emerald, the green variety of Beryl, is the most famous and valuable green gemstone. Its beautiful green color, combined with durability and rarity, make it one of the most expensive gemstones. Deep green is the most desired color in Emeralds. In general the paler the color of an Emerald, the lesser its value. Very pale colored stones are not called Emeralds but rather "Green Beryl". They are sometimes heat treated, which causes their color to turn blue and transform into Aquamarine.