The term nonrenewable resource is generally used to refer to the energy-providing resources of oil, coal, natural gas and uranium. Nonrenewable resources are any resources comprised solely, or in part, from elements on the periodic table and that were created through chemical reactions that either required conditions no longer found on Earth or that took millions of years to complete, making them nonrenewable in a practical sense.
Coal is produced when the pressure on carbon minerals forces them to fossilize. By definition, fossilization takes millions of years, making coal nonrenewable on a human time scale. Oil also requires fossilized organisms to create the carbon-based liquid. Natural gas is also a fossil fuel created by exposure to the intense heat and pressure of the Earth near to the core for millions of years.
Unlike the fossil fuels, uranium wasn't produced through a chemical reaction on Earth. Rather, it's produced by the heat of an exploding supernova star and was then brought to Earth during the planet's formation and later from meteors or meteorites. Although small amounts uranium are present in many areas, large amounts of uranium are difficult to find in high concentrations. Other elements, such as tin, mercury, silver, gold and cadmium are nonrenewable in the same way as uranium, but they aren't considered resources in the same way as fossil fuels.