 Urethral plugs in cats are usually composed of struvite crystals and organic matter.Urate (C5H4N4O3) stones, usually ammonium urate (NH4·C5H4N4O3) or sodium urate monohydrate (Na·C5H4N4O3×H2O), form in an acidic to neutral urine.They are usually small, yellow-brown, smooth stones.