All organic matter contains carbon. Coal is deposited in the Earth's interior. It developed about 300 million years ago from plants in a geological period which is also called Carboniferous. During the combustion of organic matter, carbon turns into the gas carbon dioxide. Dissolved in water, it becomes the so-called carbonic acid. Carbon dioxide is an incombustible, colourless and odourless gas that is easily dissolved in water. With various metal oxides or hydroxides it forms two types of salts: the carbonates and the hydrogen carbonates. As calcium carbonate it is contained in natural products such as chalk and egg shells. Specific forms of carbon, called modifications, are graphite and also the particularly valuable diamond.