These mineral samples are potassium feldspar, a common mineral in Earth’s crust. Radiometric dating by the radioactive decay of potassium-40 (K-40) atoms is one of the most commonly used techniques. Because potassium is incorporated in common minerals like feldspar and mica, this method is used to date very old samples of igneous and metamorphic rocks. The K-40 decays to form argon-40 (Ar-40) and does so with a half-life of 1.25 billion years. This method is used to date ancient rocks because of the long half-life of potassium-40 and because potassium is so common in many rocks.

The decay of Uranium-238 (U-238) to Lead-206 (Pb-206) is commonly used on grains of the mineral zircon because zircon readily incorporates uranium but rejects lead. Therefore, any lead found in a zircon crystal must have come from the decay of uranium instead of being present when the mineral crystallized. Uranium-238 has a half-life of 4.3 billion years, so this method can be used to test mineral grains from ancient times. While this method is not as commonly used for testing very old rocks, as is the K-Ar method, U-Pb dating is used to date zircon grains in ancient igneous materials. The oldest zircon ever found on Earth was found in the Jack Hills of Western Australia and was dated at 4.2 billion years old.

Carbon-14 is an isotope of carbon that is continuously produced in Earth’s upper atmosphere by cosmic rays. It then becomes part of atmospheric carbon dioxide, and is taken up by plants during photosynthesis and incorporated into plant tissue. Later, it becomes incorporated into the bodies of animals that eat the plants. After living things die and decay, the carbon-14 undergoes radioactive decay to nitrogen-14. The half-life of carbon-14 is 5,730 years. Because carbon-14 has a half-life of only 5,730 years, it cannot be used to date materials older than 50,000 years old—after that only a tiny fraction of the original C-14 remains, and the test can no longer work. Also, carbon-14 dating is only applicable to materials that were once living things and it cannot be used to date rocks.