Floating feces, grainy bits separate and sink.. Please Help!!

Hi. For months have had very odorous stool, that floats. Sometimes it doesn't always flush. When it remains in the toilet (accidentally) it separates, the 'bulk' remains floating but bits like coarse ground pepper sinks. The portion that sinks is roughly 1/4 to 1/3 of original stool. Most often the stool is very dark.
This is very unsettling.. does anyone have any ideas what is causing this? The grainy bits are angular in shape, not smooth/round. Sometimes the stool has mucous. Please help! Tweet