zircon

Those crystals in Australia, called zircons, form only where there is plate subduction, and subduction can happen only on a planet with active plate tectonics.

The zircons represent a rare report from the mysterious Hadean, the geological time period that ended about 4 billion years ago, 500 million years after Earth’s formation.

And although debate continues about the timing of the Moon-forming impact, hardy zircon crystals in Moon rocks dated last decade did suggest it could have happened at the same time.

Scientists use tiny minerals called zircons as geologic timekeepers.

The researchers' study of the zircon showed that, in several ways, its chemical fingerprint matches those of some of the oldest rocks on the planet found in West Greenland's North Atlantic Craton.