Kidney stones are hard collections of salt and minerals often made up of calcium or uric acid. They form inside the kidney and can travel to other parts of the urinary tract. Stones vary in size. Some are as small as the period at the end of this sentence — a fraction of an inch. Others can grow to a few inches across. Some kidney stones can become so large they take up the entire kidney. A kidney stone forms when too much of certain minerals in your body accumulate in your urine. When you aren’t well hydrated, your urine becomes more concentrated with higher levels of certain minerals. When mineral levels are higher, it’s more likely that a kidney stone will form.