A kidney stone begins as tiny crystals forming inside a narrow renal tubule. These tubules are small channels inside the kidney that carry fluid as the kidney filters blood. The fluid normally keeps minerals dissolved, but when the balance changes, some minerals begin to stick together. Calcium and oxalate are the most common pair. When their concentration rises or when fluid flow slows down, the minerals start to settle on the tubule wall. This creates the first tiny crystal that acts like a se