Gallstone. Close-up of a gallstone removed from person's gallbladder. Gallstones are most commonly hard deposits of cholesterol that form due to an imbalance between bile and cholesterol levels. Stones can also arise from bilirubin and calcium salts found in bile, which is produced by the liver and then stored by the gallbladder until it is needed. If a gallstone blocks one of the gallbladder's ducts, it can cause irritation and inflammation.