Cats have highly concentrated, acidic urine that predisposes them to form stones of the urinary tract, especially when certain minerals or other substances are abundant or deficient. This may be a result of diet, inflammation or infection, or possibly the pH level of the individual cat’s urine.
The urolith itself is made up of a hardened center, with various layers that have formed around it. It is unclear what starts the process of formation, but it is likely that an excess of urolith-forming minerals in the urine form small crystals that stick together to form a stone. Once the stone is formed, it gradually increases in size—a process that can take weeks or months.
Cats can also form small stones that resemble sand or toothpaste. Small stones, crystals, and other components of urine can create "plugs" that can block the urinary tract, especially the urethra.
Veterinarians identify stones based on their makeup. In cats, the most common stones are generally composed of either magnesium ammonium phosphate (known as struvite) or calcium oxalate. Calcium oxalate stones are now the most common stone found in the urinary tract and kidney, although their cause is unknown. Some types of diets used to control struvite stones may contribute to the formation of calcium oxalate stones.