Gallstones are hard, pebble-like formations that develop in the gallbladder, and most of them are made primarily of hardened cholesterol. In developed countries, over 85% of gallstones fall into this category. The rest are composed mainly of bilirubin (a waste product from the breakdown of red blood cells), calcium salts, or some combination of multiple materials. Stones can be as small as a grain of sand or as large as a golf ball, and your gallbladder can produce one large stone, hundreds of tiny ones, or a mix of both.
Cholesterol Stones
Cholesterol gallstones are the most common type by a wide margin. They’re usually yellow-green in color and are classified as cholesterol stones when at least 50 to 60% of their weight comes from cholesterol. But they’re rarely pure cholesterol. Embedded within them you’ll typically find mucin (a gel-like protein your gallbladder produces), calcium salts of bilirubin, phosphate, carbonate, and palmitate, plus trace amounts of other substances. When researchers analyze the elemental makeup, the dominant elements are carbon and oxygen, with very small quantities of calcium, sodium, magnesium, copper, and silicon.
These stones form when bile, the digestive fluid stored in your gallbladder, contains more cholesterol than it can keep dissolved. The excess cholesterol crystallizes into tiny plates that clump together, often around a core of pigment (a calcium-bilirubin complex). A thick mucus layer produced by the gallbladder lining helps trap these crystals and accelerates the process. Over time, the crystals pack together into a solid stone.
Pigment Stones: Black vs. Brown
Pigment stones are dark in color and made primarily of bilirubin rather than cholesterol. They account for a smaller share of gallstones in Western countries but are more common in parts of Southeast Asia. What many people don’t realize is that pigment stones come in two distinct varieties with very different causes.
Black pigment stones form inside the gallbladder itself and result from excess bilirubin in the bile, often due to conditions that cause rapid breakdown of red blood cells (hemolysis). They have an amorphous, tar-like structure. Their main component is a polymerized, oxidized form of calcium bilirubinate, and they frequently contain crystalline calcium phosphate or calcium carbonate. They can also harbor trace metals found naturally in bile. These stones tend to be small and hard.
Brown pigment stones have a distinctly layered, laminated appearance. They form in the bile ducts rather than the gallbladder and are strongly associated with bacterial infections or parasitic infestations of the biliary system. Bacteria in the bile ducts produce enzymes that break down fats in bile, releasing fatty acids like palmitic and stearic acid. These fatty acids combine with calcium to form insoluble “soaps” that become a major component of the stone. Brown stones also contain unpolymerized calcium bilirubinate, unconjugated bile acids, and variable amounts of cholesterol. They’re softer and greasier than black stones and are sometimes found after gallbladder removal surgery, forming on their own in the bile ducts.
Less Common Stone Types
A detailed classification study of over 800 gallstones identified several rarer types beyond the well-known cholesterol and pigment categories. Calcium carbonate stones, made primarily of carbon, oxygen, and calcium, were actually the third most common type in that study, with 139 cases out of the total. Phosphate stones (composed of carbon, oxygen, calcium, and phosphorus) and calcium stearate stones (containing carbon, oxygen, calcium, and often sulfur and copper) were far less frequent, numbering 12 and 9 cases respectively.
Protein stones, made largely of carbon, oxygen, nitrogen, sulfur, and calcium, were found in only 3 cases. Cystine stones, with a similar elemental profile, were the rarest of all, with just a single case identified. These uncommon types are curiosities rather than conditions most people will encounter, but they illustrate that gallstones aren’t a one-size-fits-all phenomenon.
Mixed Stones
Not every gallstone fits neatly into one category. Mixed stones contain two or more materials in roughly equal proportions, so no single component dominates. In the same classification study, 129 out of 806 stones fell into this mixed category. A stone might contain substantial amounts of both cholesterol and calcium bilirubinate, or a blend of pigment and calcium carbonate. Mixed stones are common enough that if you’ve had a gallstone analyzed, there’s a reasonable chance it contained a combination of materials rather than being purely one type.
Why Composition Varies
The type of stone you develop depends on what’s happening in your bile chemistry. Cholesterol stones are linked to bile that’s oversaturated with cholesterol, a situation influenced by factors like obesity, rapid weight loss, high-estrogen states (pregnancy, hormone therapy), and genetics. The gallbladder’s own mucus production plays a role too: proteins and mucin in the gallbladder concentrate faster than the bile itself, creating a sticky environment where crystals nucleate more quickly.
Black pigment stones are driven by excess bilirubin, so they’re more common in people with chronic hemolytic conditions like sickle cell disease or in those with liver cirrhosis. Brown pigment stones are fundamentally different because bacteria are doing the heavy lifting. Infections introduce enzymes that alter bile chemistry, generating the fatty acid soaps and unconjugated bilirubin that form the stone’s building blocks.
Geography matters too. In Western populations, the dominance of cholesterol stones reflects diets and metabolic patterns that favor cholesterol supersaturation in bile. In parts of Asia, pigment stones have historically been more prevalent, though that balance is shifting. Japan, for example, has seen its proportion of cholesterol stones rise as dietary patterns have changed, with gallstone prevalence reaching about 10% of the population.
What a Stone Looks Like
If you’re curious about what came out of your gallbladder after surgery, color is the quickest clue. Cholesterol stones are typically yellow-green, sometimes with a waxy or crystalline appearance. Black pigment stones look exactly as the name suggests: dark, sometimes almost black, and tend to be small and brittle. Brown pigment stones are earthy brown, softer, and greasy to the touch due to their fatty acid content. Calcium carbonate stones are pale, often white or grayish. Mixed stones can be any combination of colors and textures depending on their ingredients.
Size varies dramatically regardless of type. Some people pass tiny stones without ever knowing they had them, while others develop stones several centimeters across that fill a significant portion of the gallbladder. The number of stones varies just as widely, from a single large stone to hundreds of small ones packed together.