A kidney stone is a small stone, usually made up of calcium crystals, that forms inside the part of the kidney where urine collects. The stone usually causes little problem until it falls into the ureter, the tube that drains the kidney into the bladder, and causes an obstruction, preventing urine from draining out of the kidney and often causing severe pain.

One of the roles of the kidney is to remove waste from the body by filtering blood and making urine. That urine flows from the kidney into the bladder through the ureter, a thin tube that connects the two. The bladder empties through the urethra, a tube much wider than the ureter. A variety of minerals and chemicals are excreted in the urine and sometimes these combine to form the beginning of a stone. Over time, this can grow from an invisible speck of sand into a stone that can be an inch in diameter or larger.