Dietary calcium is great for preventing stones, actually. Calcium is bound to a couple different things that cause stones, but the body actually makes those things specifically to bind with calcium. When it happens where it is supposed to, this is a good thing. If you are low on calcium, these things get flushed, and may get trapped in the kidney. Then any calcium that passes through may bind to it. Having higher calcium intake helps prevent them from building up in the kidneys to begin with. Though extremely high amounts of calcium from vitamin supplements etc can increase the risk of getting stones, but high calcium diet is one of the best defenses against them.