Bladder stone. Coloured scanning electron micrograph (SEM) of the surface of a fractured bladder stone. Bladder stones form when substances crystallise out of urine in the bladder. They may occur due to an infection, or an obstruction that prevents the bladder from emptying fully. This sort of obstruction is often caused, in men, by an enlarged prostate. The stones may cause discomfort when urinating, with larger stones possibly blocking the flow of urine. Treatment is with drugs or ultrasound to dissolve the stones, or in the case of the large stones, surgical removal.