Whats the brown stuff in the toilet? (PLEASE READ DETAIL)?

Well, i was remodeling my bathroom, and i saw this stuff in the big tank, behind the toilet.. and i dont know what this stuff was. I asked my mom and she said that it was ewither bodily waste or rust. I hope its rust. I noticed my moms bathroom toilet has the same stuff. Does nayone know what this stuff is?? I have seen it before but never wanted ot know what it was until now. I'm not saying the stuff in the toilet BOWL, i'm talking about the toilet TANK!!!