Kidney stones are small masses that are made up of very small crystals. When the urine that is being filtered through the kidneys contains too much of a certain type of crystal, the crystals may start to join together and come out of solution, forming tiny stones. This process may take several weeks to months to fully occur. The most common type of kidney stone is made of calcium and oxalate; other types of stones include uric acid (the same type of crystal that causes gout), calcium phosphate, struvite, and cystine. While poor fluid intake and low urine output are generally considered risk factors for developing kidney stones in general, each type of stone has different predisposing factors. For example, calcium oxalate stones are more common in people who have high blood calcium levels (such as from an overactive parathyroid gland), consume lots of high-oxalate foods (such as spinach), and those who have inflammatory bowel disease or have had intestinal surgery.