You can thank meteorites for most of the gold in Earth's crust, reports the BBC. In fact, 22 billion billion tons of asteroid material that showered our planet 3.9 billion years ago were responsible for the gold, platinum, and other precious metals there (and, by extrapolation, for Glenn Beck's Goldline infomercials). The results of the new study come from examining a different metal, tungsten: By looking at different tungsten isotopes in ancient and modern rocks in Greenland, scientists were able to determine when the rocks received a dose of that metal—and, at the same time, gold.
The scientists explain that as the planet was forming 4.5 billion years ago, the gold, tungsten, and other precious metals would have followed the path iron took and sank to the core, leaving the Earth's crust free of those goodies. Instead, we have about 1,000 times more gold than there should be in the mantle. Though meteorites had been floated as a possible delivery vehicle in the past, scientists hadn't been able to prove it until now because the proportions of gold in rocks is tough to measure. Turning to tungsten did the trick. (Read more gold stories.)