Kidney stones, or renal calculi, are the precipitates of substances such as uric acid, calcium oxalate, calcium phosphate, and magnesium phosphate that usually form in the renal pelvis. A stone passing into a ureter can cause very severe pain. Approximately 50 percent of kidney stones pass from the body on their own. Stones were once removed surgically, but most are now shattered with sound waves in a procedure called lithotripsy. Stones may form in the ureter or bladder, in addition to the kidneys.