Carbon is named from the Latin carbo, charcoal, an element of prehistoric discovery that is widely distributed in nature. It is found in abundance in the sun, stars, comets and atmospheres of most planets. Carbon is found in nature in three allotropic forms; amorphous, graphite and diamond. Contrary to what television ads would have one believe, diamonds are not rare, but exist in abundance. In combination, carbon is found as carbon dioxide in the atmosphere of the earth and dissolved in all natural waters. It is a component of great rock masses in the form of carbonates of calcium (limestone), magnesium and iron. Coal, petroleum and natural gas are chiefly hydrocarbons. Carbon is unique among the elements in the vast number of variety of compounds it can form. There are upwards of a million or more known carbon compounds, many thousands of which are vital to organic and life processes. Without carbon, the basis for life would be impossible.