The gallbladder is a small, pear-shaped organ beneath the liver that stores bile, a digestive fluid produced by the liver. Bile is composed of water, cholesterol, bile salts, and bilirubin, a pigment resulting from the breakdown of red blood cells. Gallstones (cholelithiasis) are hardened deposits of this digestive fluid that form within the gallbladder. These stones can range in size from a grain of sand to a golf ball and are a common medical condition, though most people experience no symptoms.
Understanding Stone Composition and Formation
Gallstones develop when bile components become unbalanced, causing substances to exceed solubility and crystallize. Gallstones are classified into cholesterol stones and pigment stones, based on whether cholesterol or bilirubin is the primary component. When bile becomes supersaturated, these components precipitate out of the solution.
The crystallization process is influenced by how frequently the gallbladder empties. Poor emptying concentrates the bile, promoting the formation and growth of hardened deposits. The stone’s color and texture are directly linked to the most abundant component in its final composition.
White and Yellow Gallstones
Pale yellow, whitish, or greenish gallstones are almost exclusively cholesterol stones, the most frequently encountered type. These stones contain at least 80% cholesterol, which gives them their characteristic light color. They often present as a single, large stone, but they can also be numerous and have a faceted surface.
Their formation is tied to an imbalance where the liver produces more cholesterol than bile salts can dissolve. Lifestyle and metabolic factors increase the risk of this supersaturation, including obesity and a high-fat diet. Rapid weight loss, estrogen-containing medications, and a family history of gallstones also contribute to excess cholesterol in the bile.
Black Gallstones
Black gallstones are pigment stones primarily composed of calcium bilirubinate, distinct from cholesterol stones. They are generally small, numerous, and fragile, appearing deep black due to oxidized bilirubin pigment. These stones typically form within the gallbladder and contain less than 20% cholesterol.
The cause of black stone formation is usually chronic overproduction of bilirubin. This occurs during hemolysis, where red blood cells break down too rapidly, seen in blood disorders like sickle cell anemia and thalassemia. The liver cannot properly process this excessive bilirubin, leading to its precipitation with calcium in the bile. Cirrhosis, an advanced form of liver disease, can also cause black gallstones.
Brown Gallstones
Brown gallstones are the second type of pigment stone. They have a softer, earthy brown, or sometimes greasy texture and are composed of calcium bilirubinate, cholesterol, and calcium salts of fatty acids. Brown gallstones are strongly associated with infection and inflammation within the biliary tract.
These stones often form in the bile ducts (choledocholithiasis), rather than in the gallbladder itself. Bacteria in the bile ducts release an enzyme that breaks down bilirubin, making it insoluble. This encourages precipitation with calcium. This process is often a sign of biliary tract stagnation or recurring infection, making them a different clinical concern than other stone types.