Bladder stone. Coloured scanning electron micrograph (SEM) of a fractured bladder stone. Bladder stones are crystalline masses that form from the minerals (ammonium phosphate or calcium oxalate) and proteins which naturally occur in urine. These types of stones are much less common than kidney stones. 95 per cent of all bladder stones occur in men, most of whom have an enlarged prostate gland or a urinary tract infection (UTI). Magnification: x500 when printed at 10 centimetres wide.