Because of the structure of the crystals, zircons never include lead when they are forged inside the Earth.
Here's how that works: The nuclei of radioactive elements decay — or spontaneously break down — at predictable rates.
Geologists have found annual layers in ice that are easily counted to multiple tens of thousands of years, and when combined with radio isotope dating, we find hundreds of thousands of years of ice layers.
Using the known rate of change in radio-active elements (radiometric dating), some Earth rocks have been shown to be billions of years old, while the oldest solar system rocks are dated at 4.6 billion years.
He created a catalogue of strata (which all got colorful names such as Lias Blue, and Ditto White) and argued that each one represented a distinct time in Earth's history — a principle known as fossil succession.
The accumulating evidence pointed to an extraordinary new idea: that the history of Earth goes back much, much further than any human memory.