What fatman6502002 said: “It might not be in true color.”
If not that, possibly olivine.
http://en.wikipedia.org/wiki/Olivine
Turns out there are 51 kinds of green stone, wow.
http://www.google.com/images?q=dark+green+stone&sa=X&oi=image_result_group
probably correct ... mineral deposit of some sort, if color is not distorted. Is soil considered a mineral deposit? <-— Am going to check that one ... LOL.