I've heard of this associated with a diet high in oxalates.
Chocolate, coffee and teas, berries, currants, kiwifruit, grapes, figs, tangerines and plums, almonds, cashews, and peanuts. Any kind of soy product. Wheat germ or bran.
Anything green and leafy:
Spinach, chard, beet greens, rhubarb...