Hi Antoine, if you could use a spectroscope you could in a minute know if it was an emerald or another gem. Also a Specific gravity test would get you closer to what the gem is. if you polished one side and did an RI this would also help Identify the gem. I would doubt if it is Emerald just by looking at your photo, I would hazard a guess at Tourmaline. But no one will positively identify a gem from a photo, unless it is a distinct Crystal formation then that may be possible. Hope you did not pay a large amount of money for it. It is not wise to purchase gems if you cannot do any field tests on the gem to have a reasonable idea on its identity.