Zircon is the "time-lords" of the earth.They are indestructible and take up radioactive materials, so they're used to track events in deep time that would otherwise be lost to us.SARAH MCCAMMON, HOST: The Earth is really old - about 4 1/2 billion years old.And when scientists want to learn about its earliest history, they turn to a mineral that serves as an almost perfect geologic clock.
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