The oldest piece of the Earth to date has been located, and it's in Western Australia. It's a fragment of a crystal in a fragment of a rock: a zircon, to be precise.
Earth's crust is full of zircons, which form when magma is forced to the surface and cools. They have such a high melting temperature that even if they are subducted, they usually don't remelt and are often incorporated into newer rocks once they surface again. The zircons in question, in Australia, have most likely been through the magma cycle a few times. However, geologists can tease apart individual fragments of a given rock, dating each one with various techniques, including individual zircon crystals.
In 2001, a team dated a zircon found in the Jack Hills at 4.4 billion years old, using radiometric dating based on the decay of uranium isotopes into lead. The result shocked a lot of people who had theorized that the Earth was still far too hot for rocks to form that soon after the formation of the Solar System (4.5 billion years ago), and so like all good scientists, they continued to analyze their data. Now they've confirmed the original dating, which means Earth did indeed cool off a mere 100 million years after it formed. It follows, then, that liquid water could not have been too far behind, and with it, life.
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Go forth and geek.