Kidney stones are solid crystals formed from the salts in urine. They are sometimes called renal calculi. Kidney stones can block the flow of urine and cause infection, kidney damage or even kidney failure. They can vary in size and location. Kidney stones can block the flow of urine, which can then cause damage to your kidneys and sometimes kidney disease. Stones increase your chance of urinary and kidney infection and can result in germs spreading into your bloodstream (septicaemia).