Causes

It's not clear what causes gallstones to form. Doctors think gallstones may result when:

• Your bile contains too much cholesterol. Normally, your bile contains enough chemicals to dissolve the cholesterol excreted by your liver. But if your liver excretes more cholesterol than your bile can dissolve, the excess cholesterol may form into crystals and eventually into stones.
• Your bile contains too much bilirubin. Bilirubin is a chemical that's produced when your body breaks down red blood cells. Certain conditions cause your liver to make too much bilirubin, including liver cirrhosis, biliary tract infections and certain blood disorders. The excess bilirubin contributes to gallstone formation.
• Your gallbladder doesn't empty correctly. If your gallbladder doesn't empty completely or often enough, bile may become very concentrated, contributing to the formation of gallstones.

Types of gallstones

Types of gallstones that can form in the gallbladder include:

• Cholesterol gallstones. The most common type of gallstone, called a cholesterol gallstone, often appears yellow in color. These gallstones are composed mainly of undissolved cholesterol, but may contain other components.
• Pigment gallstones. These dark brown or black stones form when your bile contains too much bilirubin.