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If you didn’t know their age, you might not see their appeal. In fact, you might not see them at all. These miniscule crystals of zircon—a mineral beloved by geologists because of its extraordinary resistance to the march of time and the interesting things that can be found inside it—are less than a half millimeter in diameter. The latticework of zirconium, silicon, and oxygen atoms that make up zircon also form cages that imprison foreign elements that get trapped inside. Those elements include radioactive uranium and the lead it decays into—grains of sand in an hourglass that allows geochemists to calculate the age of the zircon and, by association, the igneous rocks within which they formed.

But the oldest zircons aren't found in an igneous rock. As the mineral most resistant to weathering, it survived long after the rest of that rock had eroded away. It then got trapped within sediments and was discovered within partly metamorphosed sedimentary rocks in western Australia. The sedimentary rocks formed roughly three billion years ago, and the zircons they contained are even older. The most ancient of them were measured at close to 4.4 billion years old—remarkable considering that the Earth itself only came together a little more than 4.5 billion years ago...

The oldest piece of the Earth, examined atom-by-atom | Ars Technica

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