Kidney stones are typically yellow or brown, with a smooth or jagged surface, and they range from the size of a grain of sand to, rarely, as large as a golf ball. Most stones people pass at home are small pebble-like objects that can look like a piece of gravel or a rough grain of sand sitting in the toilet. What a specific stone looks like depends on what it’s made of, how long it’s been growing, and where it formed inside the kidney.
General Size and Shape
Kidney stones range from less than 2 millimeters (smaller than a grain of rice) to over 2 centimeters in diameter. Most stones that cause symptoms fall somewhere in between. Stones under 2 mm have an 85% to 90% chance of passing on their own. Once a stone reaches 6 to 10 mm, those odds drop to 30% to 50%, and stones larger than 10 mm almost always need medical intervention to remove.
Shape varies just as much as size. Some stones are round and smooth, almost like tiny pebbles. Others are rough, jagged, and spiky, with irregular edges that can scratch the lining of the ureter as they move through. The jagged ones tend to cause more pain during passage.
How Different Stone Types Look
About 80% of kidney stones are made of calcium oxalate, and these are the ones most people picture. They tend to be dark brown or black with a rough, bumpy surface. Some have a spiky, crystalline texture that makes them look almost like a tiny piece of coral.
Uric acid stones look quite different. They’re usually smooth, round or oval, and range from yellow to reddish-brown. They form when urine is consistently acidic and tend to have a waxy appearance compared to the rough calcium stones.
Struvite stones are often pale, yellowish-white, and can grow very large very quickly because they’re fueled by urinary tract infections. These are the stones most likely to become “staghorn” stones (more on those below). Cystine stones are relatively rare and tend to be yellow or light pink with a waxy, somewhat translucent look.
What Staghorn Stones Look Like
The most dramatic-looking kidney stones are staghorn calculi. These are branching stones that grow to fill the internal collecting system of the kidney, taking on a shape that resembles deer antlers or a piece of coral. The branches extend into the funnel-shaped structure that collects urine and into the smaller chambers branching off from it. A complete staghorn stone can be several centimeters across and essentially becomes a cast of the kidney’s interior plumbing. These stones are far too large to pass naturally and require a surgical procedure to break up and remove.
What You See in Your Urine
Before you ever see a stone, your urine may change. Blood in the urine is one of the most common signs, and it can make your urine look pink, red, or dark brown. Some people notice cloudy or foul-smelling urine instead. Occasionally, you might see tiny bits of sandy grit or sediment in the toilet, which are either small stones or fragments of a larger one breaking apart.
If you’re actively passing a stone, your doctor may ask you to strain your urine through a fine mesh filter so you can catch the stone. This is worth doing because analyzing the stone tells you exactly what type it is, which directly determines how to prevent the next one. The stone you catch might be as small as a grain of sand or as large as a small pea, and it may be hard to spot if it’s tiny.
How Stones Appear on Medical Imaging
If you go to the emergency room with kidney stone symptoms, you’ll likely get a CT scan of your abdomen and pelvis. On a CT scan, most kidney stones show up as bright white spots against the grey tissue of the kidneys and the tubes leading to the bladder. This makes them relatively easy to spot, and the scan can pinpoint the stone’s exact size and location.
Ultrasound can detect stones inside the kidney itself, but it often misses stones that have already moved into the ureter (the tube connecting the kidney to the bladder). A plain X-ray can catch larger calcium-based stones but is unreliable for uric acid and cystine stones, which don’t show up well on standard X-rays. This is one reason CT scans are the preferred tool: they catch virtually every type and size.
What Crystals Look Like Under a Microscope
When a lab analyzes your urine sample, technicians look for crystals that signal stone formation. Each type has a distinctive geometric shape. Calcium oxalate crystals appear as tiny pyramid shapes or dumbbell shapes, depending on the specific form. Uric acid crystals are rectangular or diamond-shaped. Struvite crystals have a distinctive “coffin lid” shape, flat rectangles with angled ends. Cystine crystals are hexagonal, like miniature stop signs.
You won’t see these shapes with your naked eye, but they’re part of what helps your doctor determine what kind of stone you’re dealing with if you haven’t caught the stone itself for direct analysis.
Where Stones Get Stuck
Most stones form inside the kidney and then travel downward toward the bladder. They tend to get lodged at three natural bottleneck points: where the kidney meets the ureter, where the ureter crosses over the pelvic bone, and where the ureter enters the bladder. A stone sitting at any of these spots causes the backup of urine and intense pain that brings most people to the doctor. On imaging, you’ll see the stone as a bright white dot at one of these junctions, often with the kidney above it looking swollen from the fluid that can’t drain past the blockage.