Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium or sometimes vanadium.[2] Beryl has a hardness of 7.5–8 on the Mohs scale.[2] Most emeralds have lots of material trapped inside during the gem's formation,[3] so their toughness (resistance to breakage) is classified as generally poor. Emerald is a cyclosilicate.