Bladder stones are hard mineral buildups that form in the bladder, usually as a result of a urological problem, such as urinary tract infection or enlarged prostate. Bladder stones occur when urine is concentrated and minerals crystallize and clump together.
Bladder stones may block the tube that carries urine from the bladder to the outside of the body (urethra). This may cause trouble urinating, painful urination, or complete inability to urinate.
WebMD Medical Reference from Healthwise