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Becky was a science reporter at Live Science and The Pasadena Star-News; she has freelanced for New Scientist and the American Institute of Physics.

She earned a master's degree in geology from Caltech, a bachelor's degree from Washington State University, and a graduate certificate in science writing from the University of California, Santa Cruz.

The radiation damage means the zircons could have been contaminated during their long lifetime.

Zircons hold minute amounts of two naturally occurring uranium isotopes — isotopes are atoms of the same element with different numbers of neutrons.

The lead hasn't wiggled around enough to throw off the ages.

A typical age measurement, made with a machine called an ion probe, zaps zircon segments that are thousands of times larger than the damage clusters.

The Australian zircons, from the Jack Hills, aren't the oldest rocks on Earth — those are in Canada — but about 3 billion years ago, the minerals eroded out some of Earth's first continental crust and became part of a riverbed.

Just three of the very oldest zircons have been found, ones that date back to almost 4.4 billion years ago.

Their extreme age always makes the dates suspect, because of possible radiation damage.

That means Earth cooled quickly enough for surface water and continental-type rocks just 100 million years after the moon impact, the massive collision that formed the Earth-moon system. ] "The zircons show us the earliest Earth was more like the Earth we know today," Valley said.

"It wasn't an inhospitable place." Dubious history Zircons are one of the toughest minerals on the planet.