Finally, bile salts latch onto cholesterol molecules, keeping them in solution. Stone Formation Should any of these biological balances backfire, the sludge-like gallbladder contents can crystallize. A stone forms when a speck of calcium becomes coated with either cholesterol or the pigment bilirubin. Bilirubin comes from the blood's oxygen-carrying molecule, hemoglobin. While pigment stones are small, dark, and relatively rare, cholesterol stones are crystalline and waxy, can grow quite large, and may accumulate in the hundreds.
Many stones are mixed, with pigment on the inside, wrapped in a cholesterol coat.
About half of people with gallbladder stones do not even know that they have them. Painless stones probably float freely in the gallbladder. Pain results when a stone is small enough to pass through and lodge in either the cystic duct leading from the gallbladder, or farther along in the common bile duct, which is shared by the gallbladder and the liver and leads to the small intestine. In fact, one large stone trapped in the gallbladder is not as likely to cause pain as are several small stones that can escape. The type and severity of symptoms depend upon where stones lodge.