Under normal circumstances, most bile is recirculated in the digestive tract by being absorbed in the intestine and returning to the liver in the bloodstream. Gallstones are pieces of solid material that form in the gallbladder. Gallstones form when substances in the bile, primarily cholesterol and bile pigments, form hard, crystal-like particles. Cholesterol stones are usually white or yellow in color and account for about 80 percent of gallstones. They are made primarily of cholesterol.
Pigment stones are small, dark stones made of bilirubin and calcium salts that are found in bile.
They account for the other 20 percent of gallstones. Risk factors for pigment stones include cirrhosis, biliary tract infections, and hereditary blood cell disorders, such as sickle cell anemia. Gallstones vary in size and may be as small as a grain of sand or as large as a golf ball. The gallbladder may develop a single, often large, stone or many smaller ones, even several thousand. Progress has been made in understanding the process of gallstone formation.