Gallstones are solid crystals made of either cholesterol or bilirubin, a pigment material.
They can range in size from almost microscopic to over half an inch in diameter. Gallstones form in the gallbladder, an organ located next to the liver whose purpose is to store bile, a digestive fluid produced by the liver to aid in the digestion of fat. Some components of fat, like cholesterol, do not respond well to bile and rather than dissolve as they should, they solidify and begin to form crystals. These crystals are gallstones. There are also some gallstones made of bilirubin, and the reason that these form is not certain.