Emerald is a gemstone variety of the mineral beryl (Be3Al2(SiO3)6). It generally has a green color (the Greek word smaragdos translates to ‘green gem’ in English), which is related to the presences of trace amounts of chromium or vanadium in the mineral structure. Emerald is a precious stone commonly used in jewelry, together with sapphire, ruby and diamond.
Information source: Mindat.org
Image: Emerald crystals found in Colombia. Source: Parent Géry, Wikimedia Commons.