Kidney stones refer to solid masses made up of tiny crystals. There can be one or more stones in the kidney or ureter at the same time. Kidney stones typically leave the body in the urine stream, and a small stone may pass without causing symptoms. If stones grow to sufficient size (usually at least 3 millimeters (0.12 in) they can cause blockage of the ureter. This leads to pain, most commonly beginning in the flank or lower back and often radiating to the groin. This pain is often known as renal colic and typically comes in waves lasting 20 to 60 minutes.