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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.