Illustration of stones in the gall bladder. Gall- stones develop when a chemical imbalance occurs in the bile that is stored in the gall bladder. Here, a large stone can be seen, and a collection of smaller stones. The shape of gallstones varies: from round to oval, smooth or faceted lumps, 1-25 mm across. Most commonly they occur when the liver puts too much cholesterol into the bile (which occurs in obesity), or fails to dissolve the cho- lesterol already there. Stones can get stuck in the bile duct, or block bile secretion. Only about 20% of gallstones cause symptoms, mainly in over- weight people. They can be removed with drugs or shock-wave treatment that shatters the stones.