Kidney stones form when: 

• Urine contains so much of certain chemicals that they turn into solids. 
• Urine doesn’t contain enough natural stone-preventing substances to stop solids from forming. 
• Urine contains more than the usual amount of a substance that is known to act as a trigger for stone formation. 

About 80% of kidney stones contain calcium. Most often, oxalate is the chemical that hooks up with calcium. The resulting stones are made of calcium oxalate. But some kidney stones are made of other chemicals. 

To help prevent kidney stones from forming: 

Drink enough water. Drinking water dilutes the chemicals in urine that lead to stones. If you have had a kidney stone, strive to drink enough liquids to pass 1½ to 2 quarts of urine a day. It may help to include some citrus drinks, such as lemonade and orange juice. The citrate in these beverages helps to stop stones from forming. 

Eat calcium-rich foods. The most common problem for people with calcium oxalate stones is that they absorb too much oxalate from their intestines. The kidneys must get rid of this oxalate. So, the concentration of oxalate goes up in the urine. The calcium in foods and drinks binds to the oxalate inside the gut. Less calcium goes out in the urine. This reduces the chance that stones will form. 

Decrease sodium (salt) in your diet. A high-sodium diet can trigger kidney stones because it increases the amount of calcium in your urine. Federal guidelines suggest limiting total daily sodium intake to 2,300 milligrams (mg). If you have had a kidney stone, try to reduce daily sodium to 1,500 mg. 

Limit animal protein. Eating too much animal protein increases the amount of uric acid in the urine. This can cause uric acid stones. It also can act as a trigger to form stones that contain calcium. A high-protein diet also reduces levels of citrate. This is the chemical in urine that helps prevent stones from forming. 

Limit stone-forming foods if you have a history of kidney stones. This applies only to people known to have passed a calcium oxalate or calcium phosphate stone. Beets, chocolate, spinach, rhubarb, tea and most nuts are rich in oxalate. Colas are rich in phosphate. Both of these can bind to calcium in the urine to form stones.