The oldest known Earth rock is a zircon crystal found in the Jack Hills region of Western Australia. It is estimated to be 4.4 billion years old, which is just a few hundred million years younger than the Earth itself.

While this zircon crystal is the oldest known rock, it's worth noting that:

* It's not a whole rock: It's a tiny fragment of a much larger rock that no longer exists.

* There might be older rocks: It's possible that older rocks exist, but haven't been discovered yet.

The Jack Hills zircon provides valuable clues about the early Earth, including the presence of liquid water and a relatively cool surface much earlier than previously thought.