Build up of fat Mark. Basically it's balls of cholesterol blocking up the gallbladder which is the part of the body that releases the bile into your stomach to break fats down. If it can't do its job it backs up, the fats can't digest and the bile backs up which in turn causes the liver to stop functioning properly. Makes you feel proper crap and can cause all sorts of issues if not sorted. I'll be glad when it's done with. In short stick to a healthy diet!Dave, what causes the stones?