Gallstones. Cutaway computer illustration showing gallstones (black) in a human gall bladder (yellow sac). The gall bladder is a small muscular sac lying under the liver (top). It expels bile (a fat emulsifier) into the intestines (not shown) through the bile duct (yellow tube). Gallstones 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. They usually cause no symptoms unless one becomes stuck in the bile duct, which can lead to acute pain, jaundice and infection. Surgical removal of the gall bladder is sometimes necessary.