Late response. However this doesn't appear to obsidian.. but more like hematite. (check to see if the stone has any magnetic energy)
And yes Obsidian can be 'man made' and have a verity of colors associated with it.. But the most common man made obsidian is green or a greenish blue..
Odd thing here is, Most if not all crystals and gems care being chemically altered or are under some sort of mass production.. It is getting harder to tell what is real and what isn't.
(buyer Beware) I would never buy obsidian tumbled. As a rock tumbler myself.. I have sound Obsidian is to brittle or soft (hardness on the low end of 5) to get an even tumble or even a good polish. Best leave these alone and look for all natural.
Just remember, Obsidian is or should appear like black glass or the more uncommon Mahogany Obsidian. Black with brown streaking.
P.S. Word of advice.. place the stone on a white or clear surface. Using 'White' or filter natural light, stand approx four feet away from the subject matter.. Take the image, (making sure that you hear the 'beep' sound which indicates that the subject is in focus) trying not to move while taking the shot (resting your arm or the camera on a table etc) transfer the image to your computer, using your software or sites like photobucket or picnik one can alter, crop and sharpen the images..