A strong green colour, slightly muted so it's a little less intense than what we call emerald, though the French name is vert emeraude. Its permanence, transparency, and lack of toxicity make it a very useful pigment.
It comes from one of the two characteristic colours of chromium compounds. Anyone who's been in chemistry classes will have seen the startling transformation by heating of a brilliant reddish-orange dichromate to the matt green of chromate. In fact that's the deeper, more bluish tinge of the powder.
Chromium (III) oxide, or chromium sesquioxide, also has this chromate colour. If you hydrate it you get it rather brighter and greener again, and this is viridian, Cr2O3.2H2O.
As a pigment for painters, it's a modern invention: discovered in Paris in 1838 by Pannetier and Binet, who kept their method secret; the chemical method patented by Charles-Edouard Guignet in 1859 uses boric acid and potassium dichromate. So another name for it is Guignet's green.
What's known in English as emerald green is a copper compound.