Onyx is a natural stone formed from dissolved limestone that redeposits. When used in the kitchen, it makes an attractive countertop. Before using your onyx countertop, it is important to seal it. After that, you should reapply the sealant every one to three years. Because onyx is a soft stone, it can become easily scratched or appear dull. Sealing the onyx also helps prevent acids from tomatoes and other food or from cleaners from etching it. Clean the surface of the onyx with a mixture of warm water and a special onyx cleaner.