Most kidney stones are yellow or brown, with either a smooth or jagged surface, and range from the size of a grain of sand to the size of a pea. They look like small pieces of gravel or rough sediment, and if you passed one into a strainer, you might mistake a tiny one for a speck of sand. Rarely, kidney stones grow as large as a golf ball.
Size: From Sand Grains to Golf Balls
Kidney stones span a surprisingly wide range. A 3mm stone, one of the most common sizes, looks like a small grain of sand. A 5mm stone is roughly the size of a pencil eraser or a grain of rice. Most stones that cause symptoms fall somewhere in this range. Stones larger than 10mm are less common but do occur, and at that point you’re looking at something closer to a small marble.
Size matters because it determines whether the stone will pass on its own. Stones 1mm or smaller pass naturally about 87% of the time. At 5mm, the passage rate drops to around 60%. By 9mm, only about a third pass without intervention, and stones 10mm or larger have roughly a 25% chance of making it out on their own. If your doctor tells you a stone is under 5mm, the odds are in your favor. Above that threshold, you may need a procedure to break it up or remove it.
Color and Texture by Stone Type
Not all kidney stones look the same. Their appearance depends on what they’re made of, and there are four main types.
Calcium oxalate stones are by far the most common, making up the majority of all kidney stones. They tend to be dark brown or black with a rough, jagged, bumpy surface. Under a microscope, the crystals that form these stones have distinctive geometric shapes: some look like tiny pyramids, others like dumbbells. That jagged texture is part of why they hurt so much passing through the urinary tract.
Uric acid stones are typically smooth and orange to reddish-brown. They form when urine is consistently acidic. Under magnification, the crystals appear as flat rectangles or diamond-shaped plates. Because they’re smoother than calcium oxalate stones, they can sometimes be less painful to pass at the same size, though “less painful” is relative.
Struvite stones are pale, off-white or yellowish, and relatively soft compared to other types. They form in response to urinary tract infections and can grow quickly. These are the stones most likely to become very large because they thrive in the alkaline environment that certain bacteria create. Under a microscope, their crystals have a distinctive “coffin lid” shape.
Cystine stones are the rarest type. They’re usually yellow or light pink with a waxy appearance and tend to be smoother than calcium stones. Their crystals form hexagonal (six-sided) shapes under magnification.
Staghorn Stones: The Largest Type
Some stones don’t stay small. Staghorn stones fill the entire collecting system of the kidney, branching out into multiple chambers until they resemble the antlers of a deer. They’re most often made of struvite, forming during chronic urinary infections. Because they grow to fill the space they’re in, no two staghorn stones look exactly alike, and they can vary enormously in how much mineral they contain. These stones are too large to pass naturally and always require surgical removal.
What Your Urine Looks Like With a Stone
Before you ever see the stone itself, your urine may give you a visual clue. A stone scraping along the lining of your ureter or bladder can cause blood in the urine, which shows up as pink, red, or cola-colored. Sometimes the bleeding is so slight it’s only detectable through a lab test, but many people notice the color change on their own. Cloudy or foul-smelling urine can also accompany a stone, especially if there’s an infection.
How to Catch and Identify a Stone
If your doctor suspects you’re passing a stone, you’ll likely be asked to urinate through a fine mesh strainer. This is the only reliable way to catch a stone at home. What you’re looking for is a small, hard particle that looks like a grain of sand or a tiny piece of gravel sitting in the mesh. It won’t dissolve in water. Small stones can be easy to miss, so check the strainer carefully each time.
Save whatever you find. Your doctor can send the stone to a lab for analysis, which identifies its chemical composition. That information is genuinely useful because it shapes the prevention strategy going forward. A calcium oxalate stone points to different dietary adjustments than a uric acid stone, for example. Knowing what type you formed is one of the best tools for avoiding a repeat episode.
Who Gets Kidney Stones
Kidney stones are remarkably common worldwide. In 2021, an estimated 106 million new cases were recorded globally. Men are consistently more likely to develop them than women across every age group, with the gap widening in older adults. The male-to-female ratio reaches above 3 to 1 in people over 85. If you’ve had one stone, your risk of forming another within the next five to ten years is significant, which is why catching and analyzing that first stone matters so much.