Zircon is the mineral containing grains considered the oldest known material formed on Earth. Tiny zircon crystals from the Jack Hills region of Western Australia have uranium-lead ages of about 4.4 billion years.
Zircon is especially valuable to geologists because its crystal structure can incorporate uranium while largely excluding lead when it forms. Uranium radioactive decay then provides a geological clock, allowing scientists to estimate the crystal’s age. Zircon is also chemically durable, so ancient grains can survive later erosion and be preserved in younger sedimentary rocks.
The Jack Hills zircons are not intact pieces of Earth’s original crust; they are individual crystals that survived from very early rocks. Researchers study their oxygen isotopes and other chemical signatures to investigate whether liquid water and relatively cool conditions existed on the young Earth.