One of the four ‘precious’ stones. An intense, deep green is the most desirable colour for emeralds. Flaws and cloudiness (‘jardin’) are very common in emeralds, so stones are often oiled, irradiated, and dyed to improve their look. Synthetic emeralds have fewer imperfections and are hard to set apart from natural stones. Emeralds belong to the beryl group of stones, which have large, perfect, six-sided crystals. Emeralds were long thought to have healing powers, especially for eyesight.