Kidney stones are accumulations of crystals that pass from the bladder through the urethra. Small stones are common and are usually passed in urine without incident. However, stones over 2mm in diameter can cause obstruction of the urethra, leading to decreased or blocked flow of urine accompanied by extreme pain and distension of the bladder until the stone passes. The stone can be formed of several different types of materials, but most are some form of calcium compound. They are very common, occurring in 10-15% of the population in the United States. They are more common in hot, dry countries.