Large gallstone (8cm in diametre) extracted from a patient. Gallstones are a hard mass composed of bile pigments, cholesterol & calcium salts. The formation of gallstones (called cholelithiasis) occurs when the physical characteristics of bile alter so that cholesterol is less soluable. Chronic inflammation of the gall bladder may be a contributory factor. They may exist for years without causing problems, or they may cause severe pain. They are treated by surgical removal of the gall bladder or the stones themselves.