A small piece of poop. Usually very hard, hence the "rock." Poop rocks can appear within any type of crap. Sometimes can be painful.
Trel forgot to flush the toilet this morning. The bowl was full of poop rocks!
When you hold your shit in for so long that your body compacts it into the only known geological formation which can cut through a diamond. It is theorized that very center of the sun contains a small core of poop rock.
I once ate three square meals a day and yet did not take a crap for an entire week. That totals up to 21 meals without shitting. When I finally did, I excreted a perfect diamond. I have yet to attain a Poop Rock.