My little girl developed dark green stools a while ago. From what I read excess iron can cause that color. I stopped the kids multivitamin I'd recently started giving her and the color returned to normal.
It can be from medication, Iron supplements, food coloring, abnormal digestion, Bacteria, Other Causes for green stool in adults are food poisoning, starvation, intestinal problem, presence of Giardia (a protozoan), irritable bowel syndrome (IBS), celiac disease, and ulcerative colitis. If any of these ailments is suspected, correct diagnosis is necessary.
I suggest seeing your doctor!