I recently completed the gallbladder. It has some little yarn ball 'stones' spelling out stones.  This organ stores bile (ugh, the thought of that makes me queasy) that is produced by the liver. Stones happen for various reasons, body chemistry, weight, and possibly diet. It is one on of those organs that can just be taken out, if its causing too much trouble. So its a funny little sack of an organ.  Take it or leave it.