Bladder stones are hard crystalized masses that form inside the bladder. This type is called a jackstone calculi because it's shaped like a toy jack. The abstract for an article in the Indian Journal of Urology desribes them:
They are almost always composed of calcium oxalate dihydrate consist of a dense central core and radiating spicules. They are usually light brown with dark patches and are usually described to occur in the urinary bladder and rarely in the upper urinary tract.
Do you think that there's one inside you right now?
-via The Soul Is Bone