This article, from Earth: Inside and Out, provides an overview of how geologists are using zircons to gain insight into the formation of our planet. It discusses how zircons are formed, their amazing durability, why they are the most reliable timepiece we have for looking at Earth's early history, the methods scientists use to study zircon, and what they have learned so far. In addition, there is information about the two ways to tell time in geology (relative and absolute), and zircons' usefulness for both. The site explains that the mineral zircon serves as a tiny time capsule, recording geologic events. It is especially useful because the oldest discovered grains (4.2 billion to 4.3 billion years old) are not much younger than the Earth itself.
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