Gallstones are hard, pebble-like deposits that range from as small as a grain of sand to as large as a golf ball. Most are yellowish-green or white, though some are dark brown or jet black depending on what they’re made of. The smallest detectable stones measure about 2 millimeters across, while the largest can fill the entire gallbladder. You might have a single large stone, dozens of small ones, or both at the same time.
Cholesterol Stones: The Most Common Type
More than 80% of gallstones in American adults are cholesterol stones. These are typically yellowish-green, pale yellow, or whitish, and they can have a waxy, slightly greasy surface. Some look almost like small, irregular pebbles you’d find on a beach. Others are smoother and more rounded. The color comes from concentrated cholesterol, the same fatty substance your body uses to build cells, hardened into a solid mass over months or years.
When a cholesterol stone is cut in half, the inside reveals a striking pattern: cholesterol crystals radiate outward from the center to the edge, like the spokes of a wheel. The crystals themselves are flat and plate-shaped under magnification. Some cholesterol stones are pure, while others are “mixed,” containing alternating layers of cholesterol and darker pigment arranged in a crescent pattern, almost like tree rings. Mixed stones tend to look darker and more mottled on the outside than pure cholesterol stones.
Pigment Stones: Black and Brown
Pigment stones form when the gallbladder accumulates too much bilirubin, a chemical your body produces when it breaks down old red blood cells. These stones come in two distinct varieties that look and feel quite different from each other.
Black pigment stones are small, dark, and irregularly shaped, often with a rough or crumbly surface. Cut one open and it has a glassy, almost obsidian-like cross section with a uniform dark interior. These stones are hard and brittle. They form inside the gallbladder itself and are more common in people with conditions that cause rapid breakdown of red blood cells.
Brown pigment stones look brownish-yellow and have a softer, greasier texture compared to their black counterparts. They’re sometimes described as clay-like or earthy. Brown stones are more likely to form in the bile ducts rather than the gallbladder, and they’re often associated with bacterial infections in the biliary system. Inside, brown stones show irregularly arranged clumps of pigment material mixed with fatty compounds, giving them a less structured appearance than cholesterol or black pigment stones.
Size, Shape, and Number
Gallstones vary enormously. A single stone can be smaller than a peppercorn or larger than a walnut. Most fall somewhere between 5 and 20 millimeters, roughly the size of a pea to a marble. Shape is equally unpredictable. Solitary stones tend to be rounder because they have room to grow evenly. When multiple stones develop together, they press against each other inside the gallbladder and develop flat facets, like a bag of dice. These faceted stones sometimes have smooth, almost polished contact surfaces where they’ve been rubbing against neighboring stones for years.
Some people have just one or two stones. Others have hundreds of tiny ones packed together. A gallbladder completely filled with small stones sometimes looks like a pouch of gravel on imaging. The number of stones doesn’t necessarily correlate with symptoms. A single small stone in the wrong position (blocking the duct that drains the gallbladder) can cause more trouble than a gallbladder full of larger ones that stay put.
How Gallstones Look on Ultrasound
Most people never see their gallstones directly. Instead, they see them on an ultrasound screen, where they look nothing like the pebble-like objects a surgeon would later remove. On ultrasound, gallstones appear as bright white spots inside the darker fluid-filled gallbladder. Behind each stone, there’s a dark stripe called an acoustic shadow, created because the dense stone blocks the ultrasound waves from passing through. This bright-spot-plus-shadow combination is the classic sign radiologists look for.
Stones also move when you change position during the scan, rolling to the lowest part of the gallbladder with gravity. This movement helps distinguish them from polyps or other growths attached to the gallbladder wall, which stay fixed in place. When the gallbladder is packed full of stones, the ultrasound may show a single bright curved line with a large shadow behind it, sometimes called a “wall-echo-shadow” sign, because the stones are so tightly packed you can’t see individual ones.
Gallstones vs. Biliary Sludge
Biliary sludge is sometimes called a precursor to gallstones, and it looks different on imaging. While stones appear as distinct bright spots with shadows behind them, sludge shows up as a hazy layer of material that settles to the bottom of the gallbladder, similar to sediment in a glass of muddy water. Sludge doesn’t cast the sharp acoustic shadow that stones do because it’s not solid enough to fully block ultrasound waves. It shifts when you move, but in a slower, more fluid way than solid stones rolling around.
Sludge is a thick mixture of tiny cholesterol crystals, calcium salts, and mucus. It can sometimes clump together into a mass that mimics the appearance of a tumor on imaging, though the fact that it moves and has no blood flow on specialized scans helps doctors tell it apart. Over time, sludge can either resolve on its own or solidify into true gallstones.
Why Color and Composition Matter
The appearance of a gallstone tells a story about why it formed. Yellowish cholesterol stones point toward metabolic factors: high cholesterol in the bile, a sluggish gallbladder that doesn’t empty well, or rapid weight loss that floods the bile with cholesterol. Black pigment stones suggest the body is breaking down red blood cells faster than normal, which can happen with certain blood disorders or liver cirrhosis. Brown pigment stones indicate a bacterial infection somewhere in the bile drainage system.
Calcium content also varies. Some gallstones contain enough calcium to show up on a standard X-ray as white spots, similar to how bones appear. These calcified stones are easy to spot, but most cholesterol stones contain too little calcium to be visible on X-ray, which is why ultrasound remains the primary way they’re detected. Interestingly, the ability to see a stone on ultrasound doesn’t depend on its calcium content, shape, or surface texture. Even soft, calcium-poor stones cast clear shadows on ultrasound as long as they’re large enough.