Kidney stone. Artwork of a kidney stone (upper left) in the ureter of a kidney (not seen). The bladder (centre, cutaway artwork) has two ureters entering it, one from each kidney. A stone (or calculus) of the kidney is usually formed by the precipitation of the mineral salt calcium oxalate from the urine. The resulting hard deposit may cause severe pain, especially when it passes down the urinary tract. A small stone may be passed out of the body spontaneously, but a large stone may block the flow of urine and must be removed. This may be done surgically or by using sound waves, applied externally, which break up the stone. The prostate gland (male) and urethra are at bottom.