Gallstone crystals. Coloured scanning electron micrograph (SEM) of the crystalline surface of a freeze-fractured gallstone. Gallstones form in the gallbladder when there is an imbalance in the chemical composition of the bile. Gallstones mostly consist of cholesterol, but can also contain calcium and bilirubin. Gallstones are usually symptomless, unless one obstructs the bile duct, causing acute pain, jaundice and infection. Treatment is with drugs to dissolve the stones or surgical removal of the gallbladder. Magnification: x750 when printed at 10 centimetres wide.