1 Calcium stones. 2  Most kidney stones are calcium stones, usually in the form of calcium oxalate. 3  Oxalate is a naturally occurring substance found in food. 4  Some fruits and vegetables, as well as nuts and chocolate, have high oxalate levels.ypes of kidney stones include: 1  Calcium stones. 2  Most kidney stones are calcium stones, usually in the form of calcium oxalate. 3  Oxalate is a naturally occurring substance found in food. 4  Some fruits and vegetables, as well as nuts and chocolate, have high oxalate levels. 5  Your liver also produces oxalate.