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Green: 1  Green stools are always normal, but they can be mistaken for black stools. 2  Bile. Most dark green stools are caused by bile. 3  Green stools are more common in formula fed than breastfed infants. It can be normal with both.  Green stools are more common with diarrhea. This is due to a fast transit time through the gut.
3 There are some dark brown almost black, hard as a rock (pebble like) small pieces in my stool for a month. Dr. Erik Borncamp Dr. Borncamp. Hard 2 say: But there are many other non-blood causes of dark stools (meds, diet, etc) . Fortunately it is easy to tell by bringing a small sample to your md to test for blood. If blood is found a colonoscopy and upper endoscopy could be performed to rule out a source of bleeding.