How do kidney stones form?

The basic problem is that the kidneys are releasing more minerals than the urine can hold in concentration. The chemistry is complex, but the root of the problem is that the urine is precipitating crystal. The kind of crystal varies with the amount of minerals excreted by the kidney, the pH of the urine, and other factors. That is why drinking a lot of water to make the urine more dilute is the first step to preventing stones in the urinary tract.