Emerald is the grass green, precious variety of beryl.   Found in Egypt, Colombia, Brazil, Russia, South Africa, Rhodesia, India, Pakistan and Zambia.   An expensive stone in the most desirable shade of grass green, in  transparent quality it is often included.  Very heavily included emerald can edge towards the opaque.   The colour comes from chromium.

Its expensive nature has meant that there are many simulants and synthetics on the market.