Now it has been found that zircon crystals in Western Australia formed around 4.4 billion years ago. From the Science Daily story:
Writing today (Feb. 23, 2014) in the journal Nature Geoscience, an international team of researchers led by University of Wisconsin-Madison geoscience Professor John Valley reveals data that confirm the Earth’s crust first formed at least 4.4 billion years ago, just 160 million years after the formation of our solar system. The work shows, Valley says, that the time when our planet was a fiery ball covered in a magma ocean came earlier.Another piece of the puzzle.
“This confirms our view of how the Earth cooled and became habitable,” says Valley, a geochemist whose studies of zircons, the oldest known terrestrial materials, have helped portray how the Earth’s crust formed during the first geologic eon of the planet. “This may also help us understand how other habitable planets would form.”