Eighty percent of gallstones are classified as either cholesterol stones or pigment stones.
Cholesterol Stones: The medical community’s current belief is that cholesterol stones are the result of bile that is made of too much cholesterol or bilirubin and not enough bile salts. Cholesterol stones may also form when the gallbladder fails to properly empty during the digestive process.
Pigment Stones: Medical science remains unsure of what causes pigment stones in the gallbladder. Those who develop pigment stones most often include people who have cirrhosis of the liver, biliary tract infections, and hereditary blood disorders that include sickle cell anemia. These are all conditions that lead to the formation of too much bilirubin.
Reference: NIH Publication No. 05–2897