Gallstone. Ultrasound scan of the gallbladder of a 33-year-old woman, with a large (15 millimetres in diameter) gallstone at centre. The gallbladder stores the digestive fluid bile, produced by the liver. Gallstone formation (cholelithiasis) occurs when gallstones, hard deposits formed of cholesterol and bile salts, form from an imbalance in the composition of the bile. Gallstones are usually symptomless, unless one obstructs the bile duct, causing acute pain, jaundice and infection. Treatment is with drugs to dissolve the gallstones, or surgical removal of the gallbladder.