A kidney stone is a hard, crystalline mineral mass that forms in the kidney when urine contains high levels of substances like calcium oxalate, or uric acid. These stones can form when there is insufficient water intake, a high-sodium diet, or an excess of animal protein. Dehydration, certain medications, and genetics also play a role in kidney stone formation. Symptoms include sharp pain, nausea, and difficulty urinating if stones block the urinary tract.