A gallstone is a solid particle that forms in the gallbladder, a small organ located just beneath the liver. The gallbladder stores bile, a digestive fluid produced by the liver that helps break down fats. Gallstones develop when there is an imbalance in the substances that make up bile, which can lead to the formation of hardened deposits. Gallstones can range in size from a grain of sand to a golf ball, and some people may have multiple gallstones at once.