Most kidney stones are yellow or brown, roughly the size of a grain of sand to a small pearl, and can be smooth, jagged, or irregular in shape. About 1 in 10 U.S. adults will deal with a kidney stone at some point, and knowing what to look for matters because your doctor will likely ask you to catch one so it can be analyzed.
Size, Color, and Texture
Kidney stones range dramatically in size. The smallest are barely visible, like a grain of sand, while others grow to the size of a pearl or even a marble. In rare cases, stones can reach the size of a golf ball. Most stones that pass on their own are under 5 millimeters, roughly the width of a pencil eraser.
Color tends to fall in the yellow-to-brown range, though this varies by composition. Some are pale tan, others a deep rust brown, and certain types have an amber or orange tint. Texture is equally variable. A stone might be perfectly smooth and round like a tiny pebble, or it might be rough and jagged with sharp edges. Those jagged edges are part of why passing a stone can be so painful, as they scrape against the narrow walls of the ureter on the way out.
How Different Stone Types Look
Not all kidney stones are made of the same material, and composition changes how they look. There are four main types.
Calcium oxalate stones are the most common, making up roughly 80% of all kidney stones. They tend to be dark brown or black with a rough, spiky surface. Under a microscope, the crystals that form these stones have distinctive geometric shapes: some look like tiny dumbbells, others like small pyramids. To the naked eye, though, they simply look like rough, dark pebbles.
Uric acid stones have a smoother, more pebble-like appearance, typically yellow to reddish-brown. An interesting feature of uric acid stones is that they can be hard on the outside but softer on the inside, because they form in layers of slightly different mineral compositions.
Struvite stones tend to be the largest of the four types. They grow quickly, often in response to a urinary tract infection, and can become quite big before causing noticeable symptoms. They’re usually lighter in color, off-white or yellowish.
Cystine stones are the rarest type. They have a waxy, amber appearance and are fairly compact. Under a microscope, cystine crystals form a recognizable hexagonal shape, which is how labs confirm the diagnosis.
Staghorn Stones
Some kidney stones grow large enough to fill the entire interior drainage system of the kidney. These are called staghorn calculi because their branching shape resembles deer antlers or a piece of coral. Rather than a small pebble you might pass naturally, a staghorn stone conforms to the shape of the space it fills, sending branches into the collecting ducts of the kidney. These stones are too large to pass on their own and require a surgical procedure to break up and remove.
What Your Urine Looks Like With Stones
Before you ever see the stone itself, your urine may give you visual clues. As a stone moves through the urinary tract, it can scratch the lining and cause bleeding. This makes urine appear pink, red, or cola-colored. Sometimes the blood is too faint to see with the naked eye, but in many cases the color change is obvious. Urine can also turn cloudy or murky when a stone is present, especially if there’s an accompanying infection.
These color changes don’t always mean kidney stones. Blood in urine has other possible causes. But if you’re experiencing flank pain alongside discolored urine, a stone is a strong possibility.
How to Catch and Identify a Stone
If your doctor suspects you’re passing a stone, you’ll likely be asked to strain your urine at home so the stone can be sent to a lab for analysis. This involves urinating through a fine-mesh strainer, similar to a small kitchen funnel, or into a glass jar. The stone will be caught in the mesh.
What you find in the strainer might not look like much. Small stones can resemble a grain of sand or a tiny piece of gravel sitting in the mesh. They may be so small you’d miss them if you weren’t looking. Larger stones are easier to spot and might look like a small pebble, yellowish-brown and either smooth or rough depending on the type. Occasionally people pass several small fragments rather than one intact stone.
Keeping any stone or fragment you catch is important. The lab analysis determines the stone’s mineral composition, which directly shapes the dietary and treatment recommendations your doctor will give you to prevent the next one. A calcium oxalate stone and a uric acid stone call for different prevention strategies, and the only way to tell them apart definitively is through that lab test.