Gallstones. Coloured X-ray cholecystogram of gall- stones (centre) filling a patient's gall bladder. The bones of the spine can be seen at right. Gall- stones are hard deposits of salts or cholesterol that form in the gall bladder when the chemical composition of bile is upset. They are most common in women, the elderly and the obese, and usually cause no symptoms unless one of the stones becomes stuck in the gall bladder's outlet duct, which causes acute pain. Hard gallstones rich in calcium show on normal X-rays, but soft gallstones may be missed. Cholecystograms are X-rays that highlight soft gallstones by staining them with iodine given to the patient by mouth.