Gallstones develop when cholesterol and other substances in the bile form crystals that become hard stones in the gallbladder. The gallbladder is a small sac located just under the liver. Gallstones can occur when your gallbladder does not empty properly or if you have too much cholesterol in your bile. Too much cholesterol in your bile is not caused by eating too much cholesterol or having high cholesterol (in the blood).
Most doctors believe that even microscopic gallstones in the gallbladder can cause symptoms. These tiny stones can form a type of sediment called biliary sludge that often can be seen on an abdominal ultrasound. For more information on an abdominal ultrasound, see the Exams and Tests section of this topic.