Did you slice it open to see if it was many stones joined together? I have read that even inside the gallbladder many stones can clump together to form a bigger one...but that it took so long to come out makes it a little suspicious in your case, doesn't it? Or maybe since it was a biggie it got stuck in the crevices of your small intestine? Wish I could be more help Sphinx!
I think that you should do a bowel cleanse! yup, uh-huh. It was stuck somewhere! (if it can get stuck, imagine what else has gotten stuck over the years!)