Kidney stones are mainly formed by calcium oxalate.

Explanation:
Kidney stones, also known as renal calculi, are solid deposits that form in the kidneys. These stones can vary in size and composition, but the most common type is calcium oxalate stones.

Formation of kidney stones:
1. Calcium: Calcium is an essential mineral that is naturally present in the body. When there is an excess of calcium in the urine, it can combine with other substances to form crystals. These crystals can then accumulate and grow into kidney stones.

2. Oxalate: Oxalate is a compound that is found in many foods, such as spinach, rhubarb, and beets. It can also be produced by the liver. When oxalate combines with calcium in the urine, it can form calcium oxalate crystals, which are the most common type of kidney stones.

3. Factors affecting stone formation: Several factors can increase the risk of kidney stone formation, including dehydration, a diet high in salt and animal protein, obesity, certain medical conditions (such as hyperparathyroidism), and certain medications.

Prevention and treatment:
1. Hydration: Drinking plenty of water is crucial in preventing kidney stone formation. It helps dilute the urine and flush out any potential stone-forming substances.

2. Diet modifications: Limiting the consumption of foods high in oxalate, such as spinach, chocolate, and almonds, can help reduce the risk of calcium oxalate stone formation. Additionally, reducing salt intake and animal protein consumption may also be beneficial.

3. Medications: In some cases, medication may be prescribed to help prevent the formation of kidney stones. These medications can help control the levels of calcium or oxalate in the urine, reducing the likelihood of stone formation.

4. Medical procedures: In severe cases, when the kidney stones are large or causing significant pain, medical procedures may be necessary to remove or break up the stones. These procedures can include shock wave lithotripsy, ureteroscopy, or percutaneous nephrolithotomy.

In conclusion, kidney stones are mainly formed by calcium oxalate. Understanding the factors that contribute to stone formation and adopting preventive measures can help reduce the risk of developing kidney stones.