About 90% of stones 4 mm or less in size usually will pass spontaneously, however 99% of stones larger than 6 mm will require some form of intervention. There are various measures that can be used to encourage the passage of a stone.

These can include increased hydration, medication for treating infection and reducing pain, and diuretics to encourage urine flow and prevent further stone formation. Caution should be exercised in eating certain foods, such as starfruit, with high concentrations of oxalate which may precipitate acute renal failure in patients with chronic renal disease.

http://en.wikipedia.org/wiki/Kidney_stone