Kidney stones can be composed of different substances that are dissolved in the urine, such as calcium, oxalate, uric acid, and cystine. Kidney stones form when there is an imbalance between the concentration of these substances and the chemicals in the urine that usually keep the substances dissolved. Frequent urinary tract infections from certain bacteria can also lead to the formation of a type of stone called struvite. Often, stones in the urinary tract (urolithiasis) are small enough to pass through the urinary system without causing any symptoms. However, bigger stones can lodge anywhere in the ureters, the tubes that lead from the kidneys to the bladder. This can block the flow of urine and cause severe pain.