Which material is called ‘black gold’? why?

Many useful substances are obtained from petroleum and natural gas. These are termed as ‘Petrochemicals’. These are used in the manufacture of detergents, fibres such as polyester, nylon, acrylic etc., polythene and other man-made plastics. Hydrogen gas obtained from natural gas, is used in the production of fertilisers such as urea. Due to its great commercial importance, petroleum is also called ‘black gold’.