(Yaakov Weiss) Most diamonds are thought to form some 150 to 200 kilometers under the surface, in relatively cool masses of rock beneath the continents. The process may go back as far as 3.5 billion years, and probably continues today. Occasionally, they are carried upward by powerful, deep-seated volcanic eruptions called kimberlites. (Don’t expect to see one erupt today; the youngest known kimberlite deposits are tens of millions of years old.) Much of what we know about diamonds comes from lab experiments, and studies of other minerals and rocks that come up with the diamonds, or are sometimes even encased within them. The 10 diamonds the team studied came from mines founded by the De Beers company in and around Kimberley, South Africa. “We like the ones that no one else really wants,” said Weiss—fibrous, dirty-looking specimens containing solid or liquid impurities that disqualify them as jewelry, but carry potentially valuable chemical information. Up to now, most researchers have concentrated on solid inclusions, such as tiny bits of garnet, to determine the ages of diamonds. But the ages that solid inclusions indicate can be debatable, because the inclusions may or may not have formed at the same time as the diamond itself. Encapsulated fluids, on the other hand, are the real thing, the stuff from which the diamond itself formed.