Its beautiful green color, combined with durability and rarity, make it one of the most valuable gemstones. Beryl also contains other, lesser known gem varieties, such as aquamarine and heliodor. Deep green is the most desired color in emeralds. The paler the color of the emerald, the lesser its value. They are sometimes heat-treated, in which they become aquamarine.
Emeralds are notorious for their flaws.
Flawless stones are very uncommon, and are noted for their extreme value. Some people actually prefer an emerald with very minute flaws over a flawless emerald, as this proves authenticity of the stone. Many emerald flaws can be hidden by treating the emeralds with oil. Newer, more effective fracture-filling techniques are also practiced. Irradiation of some emerald gems is somewhat effective in removing certain flaws.