They aren’t really stones. They’re pieces of solid material that form in the gallbladder, a small organ located under the liver.
You might not even know you have them until they block a bile duct, causing pain that you need to get treated right away.
The two main kinds are:
Cholesterol stones. These are usually yellow-green in color. They’re the most common kind, accounting for 80% of gallstones.
Pigment stones. These stones are smaller and darker. They’re made up of bilirubin, which comes from bile, a fluid your liver makes and your gallbladder stores.