Two principal types of stone, the cholesterol stone and the pigment stone, form in the biliary tract and gallbladder. Although cholesterol stones may be calcified, they are seldom visible on plain abdominal x-rays. The pathogenesis of pigment gallstones is not completely understood. Black pigment stones are seen most often in patients with cirrhosis or hemolytic anemia and are found predominantly in the gallbladder. Brown pigment stones, which are common in Asians, are the most common stone to appear de novo in the bile duct and are associated with biliary tract infection.
Pigment stones are often radiopaque and can be seen on plain abdominal x-rays.
It is insoluble in water and precipitates unless it is maintained in solution by bile salts. In micelles, the negatively charged hydrophilic ends of the molecules face outward, toward the water, and the uncharged hydrophobic regions face the center of the sphere, or toward one another. Cholesterol molecules are enclosed in the hydrophobic interiors. A pure bile salt micelle needs 50 molecules to enclose a single molecule of cholesterol. The intercalation of phospholipids, principally lecithin, between bile salt molecules improves the efficiency with which micelles solubilize cholesterol.