Kidney stones are hard, pebble-like pieces of material that are usually yellow or brown. They can be smooth or jagged depending on their composition, and they range from the size of a grain of sand to the size of a pea or larger. What a stone actually looks like when you pass it depends on which minerals formed it, how long it grew inside your kidney, and how large it got before it moved.
General Size and Shape
Most kidney stones are small enough to pass through your urinary tract on their own. Stones between 1 and 3 millimeters often pass without intervention. At 4 to 6 millimeters, they become more difficult to pass and more painful. Stones 7 millimeters or larger usually need medical treatment to remove.
To put those numbers in perspective, a 5mm stone is roughly the size of a pencil eraser. It’s a bit bigger than a grain of rice but smaller than a small pea, or about half the size of a standard aspirin tablet. Stones at the smaller end of the range can look like coarse sand or tiny gravel in your urine, which makes them easy to miss if you’re not straining your urine to catch them.
Color and Texture by Stone Type
The minerals that make up a stone determine its color, surface texture, and overall shape. Since roughly 80% of kidney stones contain calcium oxalate, that’s the type most people will encounter. But other compositions look distinctly different.
Calcium Oxalate Stones
These come in two varieties that look quite different from each other. The monohydrate form tends to be dark brown or black with a relatively smooth, flat surface. Under magnification, these crystals have a dumbbell shape and stack in layers with a waxy appearance. The dihydrate form is lighter in color, often tan or pale yellow, and has a rough, jagged surface with sharp points. Its crystals grow in a pyramid or envelope shape, which is why these stones feel spiky and often cause more pain as they pass through the ureter.
Uric Acid Stones
Uric acid stones are typically smooth, round, and range from yellow to reddish-brown. They tend to be slightly softer than calcium stones and can have a waxy or greasy-looking surface. Under a microscope, uric acid crystals appear as flat rectangles or diamond-shaped (rhomboidal) plates. These stones sometimes form in clusters, giving them an irregular, lumpy appearance.
Struvite Stones
Struvite stones form in response to urinary tract infections and can grow rapidly, sometimes becoming very large. They are usually off-white, yellowish, or light brown. Under a microscope, struvite crystals have a distinctive “coffin-lid” shape. What makes struvite stones truly unique is their ability to grow into a branching formation called a staghorn calculus. These large stones fill the collecting system of the kidney, and their shape resembles a piece of coral or a set of deer antlers. A staghorn stone can occupy the entire interior of the kidney and typically requires surgical removal.
Cystine Stones
Cystine stones are the rarest common type and tend to be yellow or light pink with a waxy, almost crystalline appearance. They can be smooth or slightly rough. Under a microscope, cystine crystals form flat hexagonal plates, which is one of the quickest ways a lab identifies this type. Cystine stones result from a genetic condition and tend to recur throughout a person’s life.
What You See When You Pass a Stone
When a stone passes, it drops into your urine and may be so small you don’t notice it unless you’re actively catching it with a strainer. Tiny stones look like grains of sand or small specks of gravel sitting at the bottom of the strainer. Larger stones are clearly visible, looking like a small pebble, irregular chunk, or jagged crystal depending on composition.
Your urine may look pink, red, or brown around the time a stone passes because the stone scrapes the lining of the ureter on its way out. You might also notice cloudy urine or a strong smell if there’s any associated infection. The stone itself may have a small amount of blood on its surface when you first catch it. Once rinsed, its true color and texture become easier to see.
If your doctor asks you to save the stone, place it in a clean container. A lab analysis can determine the exact mineral composition, which is far more useful than visual identification alone. Knowing what type of stone you formed helps guide dietary changes and prevention strategies to reduce your risk of forming another one.
Stones That Stay in the Kidney
Not all stones cause symptoms or try to pass. Some sit quietly in the kidney for months or years, growing slowly. These stones may only be discovered incidentally on imaging done for an unrelated reason. On a CT scan, kidney stones appear as bright white spots because of their high mineral density. On ultrasound, they show up as bright echoes with a dark shadow behind them. The size, location, and number of stones visible on imaging help determine whether treatment is needed or whether monitoring is enough.
Staghorn stones are the extreme version of this. Because they form inside the kidney and branch into multiple chambers, they can grow to several centimeters across without ever entering the ureter. By the time they cause symptoms, they may be too large for any approach other than surgery.