Passed kidney stones are usually small, hard pebbles that range from the size of a grain of sand to about the size of a pea. They’re typically yellow or brown, though they can also appear black, gray, or occasionally orange or pink. Most stones that pass on their own are under 5 or 6 millimeters, so what you find in your toilet or strainer may be surprisingly tiny for the amount of pain it caused.
Size, Shape, and Color
The NIDDK describes kidney stones as ranging from a grain of sand to a pea, with rare cases reaching the size of a golf ball (though stones that large almost never pass naturally). The surface can be smooth and rounded or rough and jagged, depending on the stone’s mineral composition. Jagged stones tend to cause more discomfort as they travel through the ureter, the narrow tube connecting the kidney to the bladder.
Color is variable. Brown and dark yellow are most common, but you might also pass a stone that looks black, tan, or grayish. Some are pale enough to be nearly translucent. Because many passed stones are only 1 to 3 millimeters wide, they can look like a coarse grain of sand or a tiny chip of gravel sitting in a strainer. If you’re not looking carefully, you might miss one entirely.
How Different Stone Types Look
About 80% of kidney stones are calcium-based, and their appearance differs from stones made of uric acid, struvite, or cystine. Knowing the type matters because it determines what dietary or medical steps can prevent the next one.
Calcium oxalate stones are the most common. They tend to be dark brown or black with a rough, spiky surface. That jagged texture is part of why they’re so painful. Some are round, others look like small irregular lumps with pointed edges.
Calcium phosphate stones are usually smoother and paler, often tan or off-white. They can appear chalky or slightly crumbly compared to the harder oxalate variety.
Uric acid stones tend to have an amber or reddish-brown color. They’re often smoother than calcium oxalate stones and can take various shapes, from flat ovals to rounder pebbles. People with gout or diets high in animal protein are more likely to form these.
Struvite stones can grow quickly and become quite large. They’re typically lighter in color, pale or off-white, and may have a smoother surface. These stones are associated with urinary tract infections.
Cystine stones are the rarest. They’re often yellow or light amber with a waxy, somewhat translucent look. Their surface tends to be smoother than calcium oxalate stones.
What Else Comes Out With the Stone
You’ll likely notice some blood in your urine before, during, or after passing a stone. This is normal. The stone scrapes the lining of the ureter and bladder as it moves, producing pink, red, or brownish urine that can last a few days to a few weeks. Small blood clots can also appear, though heavy bleeding or large clots are not typical and warrant a call to your provider.
Sometimes a stone breaks into multiple fragments before it exits, so you might pass several small pieces over a few days rather than a single intact stone. After procedures like shock wave lithotripsy, which breaks stones into smaller bits, you may see what looks like coarse sand or fine gravel in your urine for days afterward.
How Long Passage Takes
The timeline depends almost entirely on size. In one study tracking 75 patients with stones in the ureter, the average time to passage was about 8 days for stones 2 mm or smaller, 12 days for stones between 2 and 4 mm, and 22 days for stones 4 mm or larger. For 95% of stones to clear, it took up to 31 days for the smallest stones and around 40 days for mid-sized ones.
Pain tends to come in waves rather than staying constant. You may feel intense cramping in your back or side as the stone moves through the ureter, then hours or days of relative calm before another episode. Once the stone drops into the bladder, the sharp pain usually subsides. Passing it from the bladder through the urethra is typically much less painful, and many people don’t even feel it happen.
Which Stones Pass on Their Own
Stones under 4 mm pass spontaneously about 98% of the time. At 4 to 5 mm, the rate drops to roughly 65 to 81%. Once a stone reaches 6 mm or larger, only about a third will pass without intervention, and stones over 6.5 mm have a spontaneous passage rate of just 9%.
Location matters too. Stones closer to the bladder (in the lower ureter) pass more easily than those stuck higher up near the kidney. Current urology guidelines suggest that stones up to 10 mm in the lower ureter can be given about 30 days to pass with medication that relaxes the ureter. If the stone hasn’t moved after that window, imaging is repeated and a procedure is typically recommended.
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 tells you exactly what the stone is made of, which is the single most useful piece of information for preventing future stones. Mayo Clinic’s patient instructions outline the basic process: urinate into a collection container, pour the urine through a fine mesh strainer, and inspect what’s left behind. Do this every time you urinate, including first thing in the morning, since the stone may pass into the bladder overnight.
Check the strainer carefully. Passed stones can be as small as a grain of sand, and their color may blend with the mesh. If you spot any particle, even one that looks like a tiny speck of dirt, save it. Place it in the specimen container your provider gave you (or a clean, dry container if you don’t have one) and bring it to your next appointment. Even a fragment is enough for the lab to determine its composition.