The oldest known Earth stuff that remains on the surface of our planet is a mineral that's been called the "Time Lord" because it's so incredibly good at keeping geologic time.

The mineral is zircon, and scientists have found bits of it that formed 4.37 billion years ago, not too long after the proto-Earth's epic collision with a Mars-sized object that spawned our moon.

Tiny crystals of zircon can look like sand, or useless crud. But don't be fooled. With a radioactive tick-tock that marks the passing of billions of years, these small but mighty minerals offer us a peek into the Earth's early development.

"They are really the best markers of Earth's time, or the history of the Earth," says Michael Ackerson, a geologist with the Smithsonian's National Museum of Natural History.

Zircon crystals originate in cooling magmas in continental crusts, along with other minerals, Ackerson says. But those other minerals tend to disappear over time.

"Most of the minerals don't survive," says Ackerson. "So things like quartz, things like feldspar – they are chemically or physically weathered and eroded to a point where they are no longer quartz and feldspar."

By contrast, zircon is very resilient – which is "one of the main reasons" that this mineral is so useful, he says.

In the face of harsh winds, crushing pressures, or high heat, these hardy crystals persist. And eventually, they can end up getting incorporated into other rocks that are still forming. That means scientists can crush up the Earth's oldest rocks, pick through the debris, and find little grains of zircon that are even older.

In the Jack Hills region of western Australia, for example, there's rock that formed from a beach 3 billion years ago. The oldest zircons ever discovered came from this rock.

Ackerson once found a zircon that's 4.32 billion years old. Zircons that old "are extremely, extremely, extremely rare, and they're the only windows we have into the earliest Earth," he says.

These days, to know a zircon's exact age, scientists can zap it with a laser like the one at a geochronology lab at Penn State University. There, Joshua Garber shows how he can place a tiny crystal into a device that blasts a little hole into it and knocks off miniscule pieces.

"And then I torture them in an argon plasma to break them down to their smallest constituents," he says, explaining that a detector counts atoms of different chemical elements.

The important ones are uranium and lead. Zircon loves uranium and will take it in as it grows, but zircon hates lead. That means if you find lead inside, it pretty surely came from the decay of uranium, which happens at a steady rate, like the ticking of a clock.

"If you were going to design a chronometer for Earth from scratch, you would basically design zircon and uranium-lead dating," says Jesse Reimink, a geologist at Penn State. "If you believed in a higher power, you'd say, 'Oh, the higher power created this mineral with this specific system because it is so perfect.'"

But looking at the chemical makeup of zircon can do more than just reveal its age or the age of its associated rock. It can also give scientists clues about the conditions that existed when that zircon originally got created.

For example, Ackerson recently looked at aluminum concentrations inside ancient zircons to infer that plate tectonics may have begun 3.6 billion years ago.

And he says scientists used to think that Earth was a hot, glowing hellscape for its first first 500 million years. But the oldest zircons found on Earth show that's not so.

"We know from just this one collection of zircon crystals that the Earth had continents, which we didn't think was possible, that were interacting with liquid water oceans," he says. "We're starting to understand how and when the continents arose, how and when the oceans arose, and how that might have helped us set the groundwork for the propagation of life on our planet."

These are big philosophical questions that require looking back into deep time, he says – which is only possible thanks to tiny zircons.

This story is part of NPR's periodic science series "Finding Time — a journey through the fourth dimension to learn what makes us tick."

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