Kidney stones in urine typically look like small, solid pebbles or grains of sand, ranging from specks smaller than a grain of rice to chunks the size of a pea. Their color depends on what they’re made of: dark brown or black for the most common type, yellow or orange for uric acid stones, and yellow or pink for rarer varieties. Many people pass a stone without ever seeing it, especially if it’s tiny, so knowing what to look for makes a real difference if you’re trying to catch one.
What Each Type Looks Like
Not all kidney stones look the same. Their composition determines their color, texture, and shape, and there are four main types you might encounter.
Calcium oxalate stones are by far the most common. They’re dark brown or black with rough, jagged surfaces that can look spiky under magnification. These are the stones most likely to cause discomfort as they pass because of those irregular edges. They feel hard and gritty between your fingers, almost like a tiny piece of gravel.
Uric acid stones are yellow to orange and can appear somewhat translucent, like a small amber bead. Their surface is generally smoother than calcium oxalate stones, though some have irregular shapes. These form when urine is consistently acidic and are more common in people with gout or high-protein diets.
Struvite stones are variable in color and tend to grow large quickly. In the kidney, they can develop into a branching “staghorn” shape that fills the collecting system. You’re less likely to pass a struvite stone naturally because of their size; they usually require medical treatment.
Cystine stones are yellow or pink with a smooth, waxy surface. They’re the rarest type, caused by a genetic condition that allows an amino acid to leak into the urine.
What Your Urine Looks Like During Passage
Before you ever spot a stone, your urine itself may change color. Stones scrape the lining of the urinary tract as they move, causing small amounts of bleeding. This can turn your urine pink, red, or brown. It takes only a tiny amount of blood to cause a noticeable color shift, so even light pink urine can indicate a stone is on the move.
You might also notice your urine looks cloudy or has a stronger smell than usual. Some people see fine, sand-like particles settling at the bottom of the toilet, sometimes called “gravel” or “renal sand.” These are either very small stones or fragments of a larger one that has broken apart. They’re easy to miss if you’re not using a strainer.
How It Feels When a Stone Passes
The most painful part of passing a stone isn’t the final exit. The worst pain happens when the stone moves from the kidney into the ureter, the narrow tube connecting the kidney to the bladder. That’s what causes the intense, wave-like flank pain most people associate with kidney stones.
Once a stone reaches the bladder, you’ve made it through the hardest part. From there, it travels through the urethra, which is wider. You may feel pinching or stinging when you urinate, or a sensation of not being able to fully empty your bladder. Some people feel a brief, sharp moment as the stone exits, while others pass it without even noticing.
Size and Your Odds of Passing It
Size matters more than anything else when it comes to whether a stone will pass on its own. Stones smaller than 5 millimeters (about 1/5 of an inch) have a 90% chance of passing without medical intervention. That’s roughly the size of a small lentil or a grain of coarse salt. Between 5 and 10 millimeters, the odds drop to about 50%. Stones larger than 10 millimeters almost always need a procedure to break them up or remove them.
Most stones that pass naturally take a few days to a few weeks to make the full journey from kidney to toilet. The timeline depends on the stone’s size, shape, and where it is in the urinary tract when symptoms start.
How to Catch a Stone in Your Urine
If you suspect you’re passing a stone, catching it lets your doctor analyze its composition and recommend ways to prevent future ones. The simplest method is using a urine strainer, a small mesh cup that fits over your toilet or that you urinate through directly. Your doctor’s office can provide one, or you can buy one at a pharmacy.
Strain every time you urinate, since stones can pass at any point. After each void, check the strainer for anything solid. Stones can be as small as a grain of sand, so look carefully. If you find one, place it on a tissue or towel and let it air dry completely at room temperature for 24 hours. Then put the dry stone into a clean, dry container.
A few things to avoid: don’t store the stone in any liquid, because moisture interferes with lab analysis. Don’t tape it to anything, since adhesive residue also causes problems. And don’t send the strainer itself to the lab. Just the dry stone in a dry container, kept at room temperature.
Signs That Need Immediate Attention
Most kidney stones, while painful, pass safely. But certain symptoms signal something more serious. Seek care right away if you experience pain so severe you can’t sit still or find a comfortable position, pain with nausea and vomiting, fever and chills alongside the pain, visible blood in your urine, or difficulty urinating at all. Fever with a kidney stone can indicate an infection behind the blockage, which requires urgent treatment.