Emerald

Emerald, the green variety of the mineral beryl, is the most famous and favored green gemstone. 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. Pure beryl is white; emerald's green color is caused by chromium impurities(and occasionally by vanadium impurities). Deep green is the most desired color in emeralds. The paler the color of the emerald, the lesser its value. Pale emeralds are not called emeralds, but "green beryl". 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 great 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.

Many fakes and doublets are known. Two pale colored stones may be glued together with a deep green paste, creating a stone resembling emerald. Faceted green glass also resembles emerald, and it may be coated with a hard substance to mask its low hardness. Synthetic emeralds are also sold to unwary buyers without them knowing the stone is synthetic. Experts can distinguish all these fakes, and it is especially important to only purchase emeralds from reliable dealers. Experts can also determine if an emerald was treated with oil to mask internal flaws. Generally, unless otherwise specified, it can be assumed that an emerald has been treated with oil.