A kidney stone is a solid piece of material that forms in a kidney. Kidney stones may be the size of sand or gravel, as large as a pearl, or even larger. A stone can block the flow of urine and cause great pain.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.