Gallstones are hardened deposits of digestive fluid that form in your gallbladder. They range in size from a grain of sand to as large as a golf ball. The most common types are cholesterol gallstones, which are composed mainly of undissolved cholesterol and appear yellowish, and pigment gallstones, which are dark brown or black stones that form when your bile contains too much bilirubin. Some people develop just one gallstone, while others develop many gallstones at the same time.
Gallstones are very common, affecting 10 to 15 percent of the U.S. population. Two-thirds as many women as men suffer from gallstones. American Indians have genes that raise the amount of cholesterol in their bile, and have the highest rate of gallstones in the United States. Mexican Americans are also at higher risk of developing gallstones. Lack of exercise is a contributing factor.