How do kidney stones form?

Kidney stones are hard objects, made up of millions of tiny crystals. Most kidney stones form on the interior surface of the kidney, where urine leaves the kidney tissue and enters the urinary collecting system.

Kidney stones can be small, like a tiny pebble or grain of sand, but often are much larger. The job of the kidneys is to maintain the body's balance of water, minerals and salts. Urine is the product of this filtering process.

Under certain conditions, substances normally dissolved in urine such as calcium, oxalate, and phosphate, become too concentrated and can separate out as crystals. A kidney stone develops when these crystals attach to one another, accumulating into a small mass, or stone.