Emerald gemstones. Emeralds are a green variety of the mineral beryl (beryllium aluminium silicate), which get their colour from traces of chromium and iron. Unlike other beryls, emeralds often contain inclusions and other flaws. Uniquely among gems, these flaws are considered part of the emerald's character and add to its value. Emeralds are rated as 7.5 out of 10 on the Mohs hardness scale, with diamond rated as 10.