The oldest known rock was formed during the Hadeon.
The oldest known rock on Earth is located in Western Australia. Scientists recently discovered an ancient crystal called a zircon and dated it to around 4.374 billion years old. Because erosion carves the land, continents drift, volcanoes erupt and seas rise and fall, finding any evidence of early Earth is quite difficult.
The crystal hails from a time known as the Hadean Eon. The Earth was highly volcanic, had a partially molten surface and was frequently struck by meteors, asteroids and comets. The planet was lifeless and extremely violent.
The zircon itself isn’t much to look at. It’s about the size of a house dust mite and located inside an existing rock. It’s not exactly the kind of thing you’d find on a casual walk in the bush.
Short answer: The oldest known rock comes from Australia and is nearly 4.4 billion years old. In comparison the Earth is around 4.54 billion years old, and the moon nearly 4.5 billion years old.
