Gallstones form when there's an excess of one of the main ingredients in bile. The excess ingredient tums to sediment at the bottom of your gallbladder or bile ducts, and the sediment gradually hardens into stones. Cholesterol stones are the most common type. Pigment (bilirubin) stones are the other.

A variety of factors may be involved in this process, including:

• Excess cholesterol. Your liver extracts cholesterol from your blood to make bile. If there's too much cholesterol in your blood, the proportions in your bile will be off. Bile needs a balance of lipids and acids to hold all the ingredients together. Any excess will fall by the wayside.

• Excess bilirubin. Bilirubin is a by product of broken-down old red blood cells. You might have an excess of bilubin if you have a blood disorder that destroys too many red blood cells, or if your liver Is impaired in some way and struggling to process its normal load of bilirubin into bile.

• Not enough bile acids (bile sats).  Certain diseases can cause bile acid malabsorption, which means that you lose bile acids in your poop. If you lose too many, your liver won't have enough left to make bile with. The lack of bile acids creates an excess of lipids (cholesterol) in your bile.

• Cholestasis or gallbladder stasis. “Stasis” means inactivity. If your bile ducts or gallbladder aren't moving bile effectively through your biliary tract, the bile is more likely to form sediment. This might be an issue with the muscles or with the chemical signaling that tells them to move.