The presence of gallstones is a medical condition known as cholelithiasis. Gallstones are solid deposits that form inside the gallbladder, the small pouch-like vessel located posterior to the liver. The liver produces a fluid called bile that is stored inside the gallbladder until it is needed to digest food. Bile flows from the gallbladder through a tube called the cystic duct, enters the common bile duct, and then empties into the small intestine to aid in fat digestion. In some individuals, bile forms crystals that eventually grow larger in size and become gallstones. Most gallstones consist of cholesterol, which is a type of fat, while other gallstones, called pigment stones, are composed of bilirubin, calcium salts, and a variety of other substances.