Gallstones are hardened masses (stones) of bile. Gallstones form when bile contains too much cholesterol, bile salts, or bilirubin. The two types of gallstones are cholesterol stones and pigment stones. Cholesterol stones are more common, accounting for nearly 80 percent of all instances of gallstones. They are usually yellow-green in color and are made primarily of hardened cholesterol. An insufficient amount of water may also contribute to the development of cholesterol gallstones. Pigment stones are small, dark stones made of bilirubin.