Picture of Kidney Stone Crystal
Kidney stones are made of salts and minerals in the urine that stick together, creating small "pebbles" formed within the kidney or urinary tract. They can be as small as grains of sand or as large as golf balls. Kidney stones are a common cause of blood in the urine and often severe pain in the abdomen, flank, or groin. One in every 20 people develops a kidney stone at some point in their life.
Image Source: Flickr / Ken Gantz
Text: MedicineNet - Kidney Stones