Emerald is a form of beryl; probably the most glamorous and desirable of the family.  The colour depends on the origin, with Colombia having dominated the market for years with the classic pure green colour, slightly tinted blue.  It is this faintly bluish tinted green which is the most desirable.  Emeralds are also usually included in some way; with fibers, internal cracks or irregular colour distribution, making the stones sensitive to pressure.  Obviously the more included the stone, the more cloudy it appears and the less lustrous.  I currently carry Brazilian emerald cabochons; the colour is strong and bears that desirable bluish tint and the clarity is pretty good.