Emeralds are a variety of the mineral beryl (Be3Al2(SiO3)6,) colored green by trace amounts of chromium and sometimes vanadium. Beryl has a hardness of 7.5 - 8 on the 10 point Mohs scale of mineral hardness. Properties determining value Emeralds, like all colored gemstones, are graded using four basic parameters, the four Cs of Connoisseurship; Color, Cut, Clarity and Crystal. The last C, crystal's simply used as a synonym that begins with C for transparency or what gemologists call diaphaneity… (More on Emerald)