Gall stone. A gall stone removed from the gall bladder of a human patient. Gall stones are made of cholesterol, bile & calcium salts. They occur when there is a change in the chemical composition of the bile, usually an excess of cholesterol. This may be triggered by infection or inflammati- on. Gall stones are often found in the overweight, and users of oral contraceptives. Some gall stones cause symptoms, usually occurring when the stones get trapped in the bile duct. Ultrasound scanning detects gall stones. Treatment involves drugs that dissolve gall stones or removal of the gall bladder in severe cases.