Emeralds are the green gem variety of the mineral beryl (Be3Al2(SiO3)6) and they are identical to other types of beryl except for their chromium and (sometimes) vanadium content: it is the chromium that gives emeralds their bright green color. The toughness (resistance to breaking) of emeralds is classified as generally poor: the Beryl has a hardness of 7,5 – 8 on 10 poin Mohs scale of mineral hardnes.
Like all colored gemstones the emeralds are graded using four basic parameters – the four Cs of Connoisseurship: Color, Cut, Clarity and Crystal. The word gemstone comes from the habe of the jewelers that, before the 20th century, used the term water as a gem of the finest water to express the combination of 2 qualities: color and crystal. In the grading of colored gemstones color is the most important criterion, crystal is considered the second criterion. They are both necessary condition.