A precious gemstone formed from the mineral beryl, with a deep green hue created by trace elements of chromium. Emeralds are graded along the same 4Cs as diamonds, with the most important being colour, divided into hue, saturation and tone. To be classified as an emeralds, rather then a green beryl, the tone must be medium to dark. Emeralds are often highly included, so are graded by eye, rather than used a 10X loupe, as per diamonds. An eye-clear emerald is considered flawless.