They are coloured by the presence of Chromium, which is very stable against light and heat.
A lot of people are a bit unsure about using emeralds as a stone in their engagement ring.
They are a 7.5 on the MOH hardness scale so not particularly resistant to abrasion when compared with diamonds, sapphires or rubies. The stone is typically brittle and senstive to high heat pressure so must be worn with care!
The preferred cut is the 'step cut' (aka emerald cut) which is where all sharp edges and corners are cut off which not only increases the stones resistance to knocks and chips, but enhances the warm satiny hue of the stone. They are often also bezel set which is added protection for the stone
Virtually all emeralds have small surface fissures, internal inclusion and fractures.. it is the nature of their formation and a sign that they are naturally occuring, rather than synthetic. So almost all are oiled as part of the lapidary process to improve their clarity and soften their internal flaws. These are generally considered acceptable treatments, whereas the use of tinted oils or polymers is not, and should be disclosed as it will effect the value of the stone.
The most well known source of Emeralds is Columbia but Brazil is another important producer.
I think they look beautiful against the white of diamonds and are one of the most eye-catching coloured gemstones around.
sources: Secrets of the Gem Trade by Richard E Wise, Gemstones of the World by walter Schumann and Wikipedia