Zircon contains the oldest known material formed on Earth, with crystals dated to about 4.4 billion years.
Zircon is a zirconium silicate mineral with the formula ZrSiO₄. Its crystal structure can accept uranium atoms but strongly rejects lead when it first forms. Because uranium decays into lead at known rates, uranium-lead dating of zircon can reveal when the crystal crystallized and can preserve evidence of later geological events.
Tiny zircon grains from the Jack Hills region of Western Australia have yielded ages of about 4.4 billion years, making them the oldest known terrestrial material. They are not intact pieces of Earth’s original crust; they are durable crystals that survived erosion, transport, burial and recycling into younger rocks.
Zircon is valuable in geochronology because it is physically tough and chemically resistant. The age evidence has also been interpreted as showing that parts of Earth’s crust and conditions involving liquid water existed surprisingly early in the planet’s history, though scientists continue to study the details.