ABOUT three fourths of all kidney stones are composed of calcium oxalate¹; most calcium oxalate stones also contain a small amount of hydroxyapatite, and 10 to 12 percent contain some uric acid.² Ten to 20 percent of stones contain struvite (magnesium ammonium phosphate) produced by a urinary tract infection with bacteria that express the enzyme urease.³ Five percent of stones are pure uric acid, 5 percent contain more than 50 percent hydroxyapatite or calcium monohydrogen phosphate (brushite), and less than 1 percent are composed of cystine. Whereas most calcium oxalate stones are less than 2 cm in diameter, struvite, . . .