The mechanism of stone formation follows the same principles as the solubility of solids in liquids. For example, if we take a glass of water and add a spoonful of salt, the salt will dissolve and the grains will no longer be visible within a few seconds. If we continue adding more salt, it will also dissolve. However, after a certain point, no more salt will dissolve, and the excess will settle at the bottom of the glass. This happens because we have added more salt than the water can dissolve, exceeding the water's solubility capacity. Similarly, if the amount of substances that form stones in the urine exceeds its capacity to dissolve them, these substances will precipitate and form stones.

In kidney stones, calcium is the most common substance, so let’s explain the stone formation process based on calcium. Under normal conditions, we assume that for every ten calcium particles in the blood reaching the nephron, one is passed into the urine. However, if two, three, or four calcium particles pass into the urine instead of one, or if fifty calcium particles enter the nephron instead of ten, and the amount of calcium passing into the urine exceeds the urine’s capacity to dissolve it, calcium will precipitate and start forming stones, just like the salt in the example.

The stone formed at this stage is very small, so we can call it a "grain of sand." If it cannot be passed out with urine—meaning it gets stuck somewhere in the tubules of the nephron—and calcium continues to accumulate on the grain of sand, it will grow large enough to block the urinary tract. At this point, kidney stone disease develops. Stones of this size are unlikely to pass on their own, and medical intervention is necessary.

In summary, the substances that form stones are also present in the urine of individuals who do not have kidney stones, but in a dissolved state. If the concentration of these substances in the urine increases, stone formation can occur.