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.
There may be several reasons, including:
Bile can be part of the problem. Your body needs bile, but if it has too much cholesterol in it, that makes gallstones more likely.
It can also happen if your gallbladder can’t empty properly.
You might not notice anything, or even know you have gallstones, unless your doctor tells you. But if you do get symptoms, they usually include: