Emerald

Emeralds use a variety of the mineral beryl coloured 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. Most emeralds are highly included, so their toughness (resistance to breakness) is classified as generally poor.

Colour
Scientifically speaking, colour is divided into three components: hue, saturation and tone. Yellow and blue, the hues found adjacent to green on the spectral colour wheel, are the normal secondary hues found in emerald. Emeralds occur in hues ranging from yellow-green to blue-green. The primary hue must, of course, be green. Only gems that are medium to dark in tone are considered emerald. Light-toned gems are known by the species name, green beryl. In addition, the hue must be bright (vivid). Gray is the normal saturation modifier or mask found in emerald. A grayish green hue is a dull green.

Source

 * http://en.wikipedia.org