Calcium forms salts with oxalic acid and with phosphate. Because these salts are extremely insoluble, they frequently crystallize to form renal stones. The most common stone, accounting for 66% to 70% of the total stones in unselected series,1 consists of mainly calcium oxalate monohydrate or dihydrate crystals. These common stones are usually small, 1 mm to 1 cm in their maximal dimension, black, radiodense, and hard. Approximately 5% to 15% of the crystals in a calcium oxalate stone are calcium phosphate in the form of apatite. Less commonly, in 1% to 5% of patients, the stones are mainly calcium phosphate, either apatite or brushite (calcium hydrogen phosphate). The formation of these stones is favored when the urine pH is abnormally elevated. Approximately 10% of calcium oxalate stones contain uric acid, and patients who form such stones pose special treatment problems.

Uric acid contains two dissociable protons; the undissociated uric acid is sparingly soluble in human urine a 37° C (98 ± 2 mg/L).2 When the urine is unduly acidic, or when uric acid excretion is abnormally high, stones can be produced. Uric acid stones are radiolucent and often so large that they fill the renal pelvis and even extend out into the branches of the calyces to form a branching staghorn stone. They are white or red because of adsorption of the pigment urochrome from urine.

Cystine, an essential sulfur-containing amino acid, is poorly soluble (e.g., urine at 37° C dissolves only 300 mg/L). Patients with a hereditary tubular defect for the reabsorption of filtered cystine excrete so much that stones form.2a Cystine stones are moderately radiopaque because of the sulfur, and, like uric acid stones, they often grow to be large. The stones are lemon-yellow, hard, and covered with sparkling, shiny crystals that resemble crystallized sugar.

Magnesium, ammonium, and phosphate ions spontaneously combine to form an insoluble triple salt called struvite. This occurs in urine only because of infection with bacteria, usually Proteus sp,3 that possess the enzyme urease. This enzyme hydrolyzes urea to ammonia and carbon dioxide, and these stones are often called infection stones. Struvite stones grow to be extremely large and frequently form staghorns. They are gray to yellow, friable, and radiopaque. Because of their size and their intimate association with urinary tract infection, struvite stones are frequently damaging to the kidneys.