EMERALD

Emeralds captivating green colour is due to the presence of Chromium and Vanadium in Beryl.  Colour and transpancy are the main attributes of the finest gems, although eye-clean examples of over a carat are rare, with most stones having a turbid 'Jardin' of inclusions and a sleepy appearance.
Most commercially available gems are treated, the industry standard and acceptable treatment being colourless oil which reduces the visibility of white surface-reaching fractures and serves to stabilise the stone. 
Notable sources include Colombia, Zambia, Brazil and Zimbabwe.