How long does it take diamonds to form?
We really don't know how long it takes. There have been attempts to try to date inclusions in different parts of diamonds, and those have largely been unsuccessful. It may be that diamonds form over periods as short a time as days, weeks, months to millions of years. Typically, as with many crystals that grow on the Earth, it's not a continuous process. The diamonds may start to grow and then there may be an interruption for some reason – a change in conditions, temperature, pressure, source of carbon, whatever—and they could sit for millions, hundreds of million of years, and then start growing again. That's part of the problem of trying to put some sort of a growth period on them; things don't always occur continuously in the Earth.
We can grow diamonds in the lab and we can simulate conditions there. But there are things we have to do to grow diamonds in the laboratory that aren't obvious as to how it happens in the Earth. In the laboratory, they're typically grown, but there's some catalyst. Some metals are often added to cause the diamonds to grow, but these same catalysts are not observed in the diamonds from the upper mantle of the Earth.
How old then are diamonds?
All diamonds, as far as we know, are quite old in the Earth. Most diamond formation probably took place in the Earth in the first couple billion years of the Earth's history. There are diamond deposits that have been discovered that are younger—the rock itself, the Kimberlite, is maybe just tens of hundreds of millions of years old. The way they date diamonds is typically looking at inclusions of other minerals in the diamond that can be radioactively dated. The diamonds themselves can't be dated. But if the mineral inclusions contain certain elements like potassium and things that can be used in a radioactive dating scheme, then by dating the inclusion in the diamond you get some sense of the age of the diamond itself. And those dates always suggest the diamonds are quite old. At least hundreds of millions of years old, but in most cases billions of years old, anywhere from one to three billion years old, a time when the earth was probably hotter than it is today and so conditions were perhaps more appropriate for diamond growth.
How old is the famous Hope Diamond on display at Smithsonian's Natural History Museum?
The Hope diamond is at least a billion years old. You don't see the original rock that carried the diamonds to the surface, but they have found some Kimberlites in India that do have evidence of diamonds in them. Those Kimberlites date to at least a billion years old. So that suggests the Hope diamond and similar diamonds found in India were brought to the surface at least a billion years ago and perhaps longer ago. So we're comfortable saying that the Hope Diamond is at least a billion years old. When you look at the age spread of most other diamonds, it's probably much older that that.
What makes the Hope Diamond so unusual?
Its size and color make it very unusual. When you think of the history of people mining diamonds, only one diamond has ever been found that has produced a dark-blue diamond the size and quality of the Hope Diamond. That gives you some sense of just how unusual and how remarkable it is. Again, I've always argued that it's as remarkable as a natural history object, as a product of the Earth, as it is a human-cut gemstone. Most of the time when people write about the Hope Diamond they start with, "Well it was found in India." Part of the point I always try to make to people is really the story began a lot sooner. Many diamonds don't ever get to that point because they just didn't survive all these things that had to have happened.