Most passed kidney stones are small, hard pebbles that range from tan or yellow to dark brown, though their exact appearance depends on what they’re made of. Some look like grains of sand, others like rough gravel, and occasionally they’re as large as a marble or bigger. If you’ve just passed a stone or are expecting to, knowing what to look for helps you find it, save it, and get it analyzed so you can prevent the next one.
Size: From Sand Grains to Golf Balls
Kidney stones span a wide range, from the size of a sugar crystal to, in rare cases, a ping pong ball. Most stones that pass on their own are under 5 millimeters, roughly the width of a pencil eraser. At that size, there’s about a 90% chance the stone will pass without any medical procedure. Stones between 5 and 10 millimeters have roughly 50-50 odds of passing on their own, and anything larger typically needs treatment to break up or remove.
The smallest stones may look like nothing more than a few grains of sand in your strainer. These are easy to miss if you’re not paying close attention. Larger stones tend to have more visible texture, jagged edges, and distinct color, making them easier to spot and identify.
Appearance by Stone Type
About 65 to 70% of kidney stones are calcium-based, making them the most common type you’ll encounter. The rest are split among uric acid stones (about 10%), struvite or infection-related stones (about 15%), and cystine stones (around 2%). Each type has a distinct look.
Calcium Oxalate Stones
These are the stones most people pass. They come in two varieties: monohydrate and dihydrate. Monohydrate stones tend to be dark brown or black with a rough, bumpy surface that can look almost spiky under magnification. They’re the harder of the two, which is partly why they’re so painful to pass. Dihydrate stones are typically lighter in color, more yellow or gold, and have a smoother, more crystalline texture. Both types are usually small and irregular in shape.
Calcium Phosphate Stones
These stones are often off-white to light tan and tend to have a smoother, chalky surface compared to their oxalate cousins. They can appear on their own (accounting for 5 to 20% of calcium stones) or show up mixed with calcium oxalate, which happens in 10 to 30% of calcium stone cases. Mixed stones may have a layered appearance, with different textures visible on the surface.
Uric Acid Stones
Uric acid stones stand out because of their color. They’re typically orange to red, sometimes quite vivid. That color comes from the crystals absorbing pigments from the natural breakdown of hemoglobin in your urine. These stones can be large and numerous, meaning you might pass several at once. In some cases, uric acid doesn’t form a single solid stone but instead passes as a red-orange gravel, almost like coarse sand with a distinct color.
Struvite Stones
Struvite stones form in response to urinary tract infections and can grow quickly and become very large, sometimes filling an entire section of the kidney in a branching shape called a staghorn. They’re usually white or off-white and have a softer, crumbly texture. Because of their size, they rarely pass on their own and usually require medical removal. If you do see fragments after treatment, they tend to be pale and chalky.
Cystine Stones
The rarest type, cystine stones are distinctive: lemon yellow with what’s been described as a sugary coating on the surface. They form only in people with an inherited condition called cystinuria. Their smooth, waxy appearance sets them apart from the rough, jagged look of calcium oxalate stones.
How to Catch and Save Your Stone
Your doctor will likely ask you to strain your urine so the stone can be sent for lab analysis. This is worth doing because knowing the stone’s composition directly shapes the dietary and medical changes that prevent recurrence. Most clinics will give you a small mesh strainer, but a coffee filter works as a backup.
Place the strainer over your toilet or urinate into it each time until you see the stone or gravel appear. Check the strainer carefully after every use. Stones are often sand-colored and small enough to miss on a quick glance. Once you find something, place it in a small plastic or glass container (a clean pill bottle or jar works fine) and bring it to your follow-up appointment. Don’t discard anything that looks like it could be a stone fragment, even if it seems too small to matter. Labs can analyze even tiny pieces.
What Passing a Stone Feels Like
The worst pain typically happens while the stone is traveling through the ureter, the narrow tube connecting your kidney to your bladder. Once the stone drops into the bladder, the intense flank and abdominal pain usually eases significantly. Passing the stone from the bladder out through the urethra is often much less painful, and some people don’t feel it at all, finding the stone only because they were using a strainer.
After the stone passes, some soreness and general discomfort can linger for a few days. This is usually from irritation and mild inflammation along the path the stone traveled. That residual tenderness typically clears up within a few days without any specific treatment.
Signs Something Isn’t Right
Even after you think you’ve passed a stone, certain symptoms warrant a prompt medical visit: a fever with chills (which can signal infection), persistent vomiting alongside pain, blood in your urine that isn’t improving, trouble urinating, or pain so severe you can’t sit still or find a comfortable position. These can indicate a stone is still lodged, an infection has developed, or there’s been injury to the urinary tract. A second stone can also begin moving shortly after the first, which catches some people off guard.