Uric Acid Stones form from high concentrations of uric acid in acidic (low pH) urine.  At low pH uric acid precipitates, while at normal pH it dissolves.  In contrast to calcium stones, this type of stone can be "dissolved" purely with medical treatment.  The cause of the elevated urinary uric acid can be multifactorial including high protein diet and gout.

Treatment includes  increasing the urine pH with potassium citrate (making the urine less acidic) in addition to increasing water intake. Citrate is converted to bicarbonate, which increases urine pH.  Occasionally, allopurinol is used to decrease production of uric acid by the body. 

High uric acid levels (Hyperuricosuria) in the urine can also lead to calcium stones.  One reason may be that small uric acid crystals form, but this is not certain.  These crystals would then form seeds on which calcium crystals can grow.  Another possibility is that dissolved uric acid somehow causes calcium oxalate to precipitate into crystals and stones.  In this setting (i.e. calcium stones with high urine uric acid levles) treatement can be with either allopurinol or occasionally potassium citrate. A low purine diet is also an important aspect of prevention.