How do the oldest zircons help scientists date early Earth?

Zircon is one of the most durable minerals in geology, and it can trap uranium atoms when it forms while rejecting lead. Because uranium decays to lead at known rates, zircon is ideal for radiometric dating. Some ancient zircons from Western Australia are about 4.4 billion years old, making them the oldest known minerals on Earth. Their oxygen isotopes and chemistry also hint that liquid water and a differentiated crust existed very early in Earth's history. These tiny grains are among the few direct survivors from Earth’s earliest crust.