Urinary stones (calculi) are hardened mineral deposits that form in the kidney. They originate as microscopic particles and develop into stones over time.
Urinary stones may contain various combinations of chemicals. The most common type of stone contains calcium in combination with either oxalate or phosphate. A less common type of stone is caused by infection in the urinary tract. This type of stone is called a struvite or infection stone. Much less common are the uric acid stone and the rare cystine stone.
Urinary stones usually arise because of the breakdown of a delicate balance. The kidneys must conserve water, but they must excrete materials that have a low solubility. These two opposing requirements must be balanced during adaptation to diet, climate, and activity. The problem is mitigated to some extent by the fact that urine contains substances that inhibit crystallization of calcium salts and others that bind calcium in soluble complexes. These protective mechanisms are less than perfect. When the urine becomes supersaturated with insoluble materials, because excretion rates are excessive and/or because water conservation is extreme, crystal