Bladder stones are most commonly made of calcium oxalate, magnesium ammonium phosphate (struvite), or uric acid. Most stones contain a mix of two or three minerals rather than a single pure substance. In a study of over 1,000 urinary stone specimens, 85% were mixed-composition stones, while only about 15% were made of a single material.
The Most Common Types
A study of 86 men with bladder stones linked to prostate enlargement, published by the European Association of Urology, found that 42% had calcium-based stones (oxalate or phosphate), 33% had magnesium ammonium phosphate stones, 14% had urate stones, and 10% had mixed compositions. These proportions shift depending on the population studied, but calcium-based and struvite stones consistently top the list.
Calcium oxalate is the single most common mineral found in urinary stones overall. It comes in two forms: monohydrate and dihydrate. The most frequent combination in mixed stones is a blend of these two forms, accounting for about 35% of all specimens in one large analysis. Calcium oxalate crystals form when calcium and oxalate concentrations in urine are high enough to bind together and solidify.
Uric Acid Stones
Uric acid stones form when urine becomes too acidic. Your body produces uric acid as a byproduct of breaking down protein, and when urine pH drops below 5.5, the urine becomes saturated with uric acid crystals that can clump into stones. Among pure single-component stones, anhydrous uric acid is actually the most common type, making up about 7% of all specimens.
People who eat large amounts of protein, particularly red meat and poultry, face a higher risk. So do people with gout, who already have elevated uric acid levels. High blood pressure and low levels of a blood protein called albumin also increase the likelihood of forming uric acid stones. The average urine pH in uric acid stone patients tends to run around 5.8, notably lower than in people with other stone types.
Struvite and Infection Stones
Struvite stones are chemically composed of magnesium, ammonium, and phosphate. They form through a specific biological process: certain bacteria that infect the urinary tract produce an enzyme called urease, which breaks down urea in urine into ammonia. That ammonia makes the urine more alkaline, and as the pH rises, magnesium, ammonium, and phosphate ions combine into struvite crystals. In humans, Proteus bacteria are the most common culprit behind this chain reaction.
These infection-driven stones tend to grow quickly and can become quite large because the bacterial process continuously feeds the conditions for crystal formation. Women are roughly 1.8 times more likely than men to develop infection stones. The higher the urine pH climbs, the greater the risk. Patients with infection stones have an average urine pH around 6.4, the highest of any stone type.
Rarer Stone Compositions
A small percentage of bladder stones are made of less common materials. Cystine stones form in people with a genetic defect that prevents their kidneys from properly reabsorbing the amino acid cystine. When too much cystine ends up in the urine, it crystallizes into stones. This condition, called cystinuria, is inherited and tends to cause stones starting at a young age.
Ammonium urate stones are another uncommon variety. They can form due to genetic conditions that cause the body to excrete excessive amounts of uric acid, or in people with liver abnormalities that allow too much ammonia into the bloodstream and ultimately into the urine. Calcium phosphate stones also occur, sometimes on their own and sometimes mixed in with oxalate crystals.
Why Stones Form in the Bladder Specifically
The bladder is particularly vulnerable to stone formation when it can’t empty completely. Urine that sits in the bladder for extended periods becomes more concentrated, giving dissolved minerals more time and opportunity to crystallize. The most common reason for this is an enlarged prostate in older men, which physically compresses the urethra and blocks full emptying. Bladder stones are far more common in men than women for this reason.
Other conditions that impair bladder emptying, such as nerve damage from spinal cord injuries or neurological diseases, also increase stone risk. Foreign bodies in the bladder, like catheters, can serve as a surface for crystals to latch onto and grow. In every case, the underlying chemistry is the same: urine that stays too long and becomes too concentrated allows whichever minerals are present in excess to precipitate out of solution and harden into stone.
Why Composition Matters
Knowing what a bladder stone is made of changes how it gets treated and how you prevent the next one. Uric acid stones, for instance, can sometimes be dissolved by making the urine less acidic through dietary changes or medication, while calcium oxalate stones are too hard and chemically stable to dissolve once formed. Struvite stones require treating the underlying urinary tract infection, or they’ll simply reform. Cystine stones call for a lifelong management strategy because the genetic condition behind them doesn’t go away.
After a bladder stone is removed or passed, the fragments are typically sent to a lab for chemical analysis using infrared spectroscopy. This identifies the exact mineral makeup and guides prevention. Someone with a calcium oxalate stone might need to watch their oxalate intake from foods like spinach and nuts, while someone with a uric acid stone would focus on reducing protein consumption and raising their urine pH. The composition of the stone is, in many ways, a diagnostic clue about what’s happening in your body.