Emerald is the most precious stone in the beryl group. Emerald is any medium to dark green beryl colored by chromium. Green beryl colored only by vanadium is not considered to be true emerald, but rather as simply green beryl. The historical green color of emerald is unparalleled in the world of gemstones and it is considered as one of the most 'precious four' of all gemstones, which also includes sapphire, ruby and diamond. Although emeralds are one of the most valuable gemstones available today, most are very heavily included, rendering their resistance to breakage as generally poor. Almost all natural emeralds contain distinct characteristic inclusions and almost all are treated with oil or resin to fill tiny fissures and cracks. It is the presence of these flaws and oil which makes it quite easy to identify and distinguish natural emeralds amongst other similar gemstones. Artificial light will expose and amplify inclusions and fractures that prove the stone to be a natural emerald. Emeralds are a medium to darker green to blue-green and sometimes slightly yellowish-green beryl. The most popular and valuable color is a slightly bluish green in a medium dark tone with strong to vivid saturation, however, too much blue can decrease the value.