We present two cases of urinary calculi treated with extracorporeal shock wave lithotripsy. The stones were disintegrated and passed out within one month. Infrared spectrometer demonstrated aragonite, a variety of calcium carbonate. One patient sufferred from gout and constipation and had been taking magnesium containing laxative for several years. Another patient never took any special drug. Hypermagnesiuria (108 & 111mg/24 hrs), hypercalciuria (250 mg/24 hrs) and alkaline urine were noted in these two patients. The other urine biochemistry including calcium, phosphate and uric acid were within normal range. To the best of our knowledge, the stone constituent, aragonite, was very unstable and magnesium cation may stabilize and stop it inverting to calcite, the stable rhombohedral form of calcium carbonate.