Calcium phosphate stones are less common than calcium oxalate.  It is common to find stones that contain both calcium oxalate and phosphate crystals.  However, usually the stone is more than 50% calcium oxalate. 

If a stone has a large content of calcium phosphate, then medical causes should be investigated.  These include hyperparathyroidism and renal tubular acidosis (RTA).  If the kidneys cannot eliminate acid produced by the body, then acidosis may ensue.  Signs of an RTA include a low serum bicarbonate (of CO2), low urine citrate, and a urine pH above 5.5. 

Patients with calcium phosphate stones are treated with citrate.  They tend to have multiple recurrent stones and loss of bone density.  Over time the kidneys can become calcified; this is known as Nephrocalcinosis.