What Causes Black Stools?

Hi sir, I am having sort of black stool which is like very rough like crushed stones coming out. This is happening since last 2 weeks, before this i used to have loose motions, but then one day i ate turmeric with water and then onwards i did not have loose motions, the stool went normal but now it is black in colour with that rough thing like crushed stones. It also itches at times, there is no pain, but it happens 3 or 4 times, in a day. It doesn't get cleared at once. Please help me, i am also having a lot of cough.