Emerald is a variety of beryl, just like aquamarine.

It is one of the four precious stones.
Chemically, it is the addition of chromium that gives it its characteristic green colour.
The Colombian emerald is considered to be the queen of emeralds: the most beautiful have an infinitely deep, intense green colour and perfect transparency. There are also very beautiful emeralds from Brazil, Zambia, Ethiopia, Russia and Afghanistan.