Interview with a diamond expert

In an effort to inform Web visitors about Crater of Diamonds State Park and Diamond Science, U-Haul conducted an in-depth interview with Mike Howard, geologist at the Arkansas Geological Commission.

U-Haul: What exactly is diamond and how did it form?

Mike Howard: Diamond is one of several natural forms of carbon. With that particular type of carbon, the structure of diamond is formed when the carbon was put under conditions of very high temperatures and high pressure. This would have been at some great depth in the Earth's mantle, probably some 30 to 50 miles deep.

U-Haul: Deep within the Earth's core?

Mike Howard: Right. Well, not within the core, but within the mantle. The mantle makes up of the greatest volume of the internal portion of the Earth.

U-Haul: How did diamonds get to the surface of the Earth after they were formed, and can we discuss the rapid cool-down process also?

Mike Howard: They were carried up by some kind of explosive-charged material that moved out of the mantle. This particular material ­ or host rock ­ which they're found in, is called a pipe where it is exposed at the surface. The surface is not where the diamonds were formed. This material, filling the pipe, rose from a deeper depth and passed through the area where these diamonds were in the mantle, picked them up and brought them to the surface. One analogy is to take a soda pop bottle filled with carbonated soda pop, shake it up and pop the cap. That kind of rapid release of pressure is what these things (diamonds) came to the surface with. This rapid release of pressure also allowed for a rapid cool-down of the material as it got near the surface. Otherwise, they might have disappeared and been dispersed as carbon.

U-Haul: What is that type of rock?

Mike Howard: There's two kinds of rock known worldwide. The first one, kimberlite, was discovered to contain diamonds around 1870. Before that time, diamond had only been mined from secondary deposits, like sedimentary deposits where diamonds had been washed out and found later. After 1870, people looked for kimberlite pipes all over the world. It wasn't discovered until the mid-1970s (in Australia) that another kind of rock called lamproite also contains diamonds. It also came from the same region of the mantle and picked up diamonds. Chemically, it's a little different, and it contains different indicator minerals, minerals people pan and look for in their search for diamond-bearing pipes. So, there's actually two kinds of rock, kimberlite and lamproite, and they have a little bit different form because they contain different amounts of gasses and so forth.

U-Haul: What happens to the carbon if it does not go through the rapid cool down process and is not brought to the surface?

Mike Howard: If it comes slowly to the surface, it goes through a physical change from a diamond to graphite as a mineral.

U-Haul: Graphite is just another form of carbon, right?

Mike Howard: Graphite's another form of carbon. There's actually four forms of carbon. We've talked about diamond and graphite, but there's also two other minerals, lonsdalelite, and chaoite. Both are carbon and they're similar to graphite in their internal structure, but they're found primarily in impact structures where a meteorite has impacted the Earth. There you have very similar conditions to the mantle and any carbon present could be converted to diamond because one of these is a high-pressure, lower temperature phase than diamond. The other is a low-pressure, higher temperature phase. Lonsdalelite has been found in meteorites in the United States. Canyon Diablo in Arizona is probably the best known. It's intergrown with diamond, so the diamond in that meteorite was probably formed by the impact of the meteorite on the surface of the Earth.

U-Haul: Are there any more natural environments where diamonds form?

Mike Howard: As for the impact structures, we don't know whether the diamonds were a natural form of carbon in the meteorite, as graphite perhaps, and when the meteorite impacted the Earth it became diamond, or whether it was there originally as diamond.

U-Haul: There's no way of telling?

Mike Howard: There's really no way of telling at this stage in our science.

U-Haul: How was the diamond-bearing kimberlite pipe discovered at the Crater of Diamonds State Park?

Mike Howard: First of all, I do have to correct you, it is not a kimberlite pipe it is a lamproite pipe.

U-Haul: Okay.

Mike Howard: And, it was first mentioned in Geological literature in Arkansas by a fella named Powell in 1842. It wasn't until 1870 that we knew that pipes contained diamonds. In 1891, the state geologist crawled around on his hands and knees across the property because they knew it might contain diamonds. He spent two days looking and did not find any diamonds. One of the local farmers who owned part of the property, John Huddleston (who fancied himself a gold hunter), found two diamonds on the same day in August 1906. He took them into town, showed them to local banker and the local banker sent them to a well-known jeweler in Little Rock who pronounced them diamonds. From then on, the diamond rush was on in Arkansas.

U-Haul: You spoke earlier on the difference between a primary and a secondary source of diamond. Did you have anything to add to that?

Mike Howard: Yes. Most people don't realize that the world's original source of diamonds was India, and they are all secondary sources. We don't know where the pipes were; they eroded completely away. After diamonds were discovered in India and mined for hundreds of years, the Spanish were exploring South America and discovered diamonds in Brazil. About the time, the South African diamonds were discovered. These were all secondary sources in gravel deposits and in riverbeds. Then, the African pipes were found. Pipe mining has become the major source even though there's probably not more than a dozen pipes in the world known to contain an economic value of diamonds.

U-Haul: So, you're saying a primary source of diamonds is an actual pipe?

Mike Howard: That's right, that's the host rock that brought it to the surface of the Earth.

U-Haul: What are the difference between black diamonds and regular diamonds?

Mike Howard: Black diamonds are really poor-grade diamonds that contain a lot of graphite inclusions. That's the way the jewelry people define them. However, there is a second type of black diamond, and they may be a variety of different colors internally, but they have a dark green, essentially black-appearing surface burned on the outside of them due to radiation. This radiation apparently took place in the mantle, so there was some radioactive source, and it burned a skin on these stones. It's very difficult, without removing that skin, to tell what the quality and color of that diamond is. I've seen a one-carat, so-called black diamond from Murfreesboro that, if you take a pen light and shine it through it or hold it up to the sun, it's actually a nice brown color and even has a tint of red. The fellow asked me to examine it to see if it was truly black diamond. It had a skin on the outside of it but it was not black. If you cut it, it wouldn't have been a black diamond.

U-Haul: Are black diamonds normally kept as black diamond and sold as black diamonds? Are they worth as much or are they as useful?

Mike Howard:In general, they're considered by the jewelry trade as industrial grade, which is worth nothing. If you go back to what the jewelry trade considers a black diamond, which is those that are full of graphite inclusions. Usually, as soon as they're examined and found to be full of inclusions, they go into a bin and are crushed to be used as an abrasive. At one time, collectors were buying up small diamonds so rapidly that a lot of sub-gem grade stones were cut and put on the market. Sometimes they're so clogged full of black inclusions, you wonder how they even managed to cut them, with diamond being the hardest mineral and graphite one of the softest.

U-Haul: How old are diamonds, and were they produced all at one time? Do they continue to be produced?

Mike Howard: First of all, you should learn something about how they're dated. When diamonds formed in the mantle, they encapsulated inclusions of other minerals in them, particularly garnet. By isotopic examination of the minerals it encapsulated, not the diamond itself, we can say with some reasonable confidence that most diamonds are between three and a half and three-billion years in age. There's only been one fairly young diamond found, and it dated about 600 million years in age. So, it appears that the formation of diamonds in the mantle was almost a one-time event. I think if we look back at the separation of the Earth into a core, mantle and crust, we see that it took place about four billion years ago. It took another half-billion to a billion years for the carbon to migrate around in the mantle and meet up with other carbon, so it could begin to form diamond. Then, you have the growth of these things in the mantle over a half a billion years.

U-Haul: So, when we find all the diamonds, that's it? There's no more?

Mike Howard: Well, the mantle is loaded with them. There's no way to access them. The pipes are natural windows to the internal structure of the Earth. They carried rocks of all types from mantle to the surface. The study of these various rocks is how we know anything about the mantle.

U-Haul: Was there anything else that you wanted to add that you thought might be interesting?

Mike Howard: There's always been a great deal of commercial interest in this area, and this is not the only diamond pipe in the region. There are several additional, much smaller pipes. The crater of diamonds is called the Prairie Creek Pipe and is an actual pipe containing diamonds. It's about the 10th largest pipe known in the world, and many of the other larger pipes are commercial. This one is not a commercial pipe but it still contains plenty of diamonds for the tourist. There's at least one company in the area that has the other pipes under lease. They continue to do initial exploration work to put together a large enough piece of property with commercial-grade diamonds on it to begin mining diamonds in Arkansas. Another thing is when you find one of these pipes, you find several dozen to a hundred pipes. Of those, maybe two or three will contain diamonds. Perhaps one will be a commercial pipe. It is thought that there's a lot of pipes in this region. Unfortunately, we have sedimentary cover that has covered the great bulk of them, and generally they're relatively small. The Prairie Creek Pipe is known to extend for 81 acres, but all other pipes in the area are five to 10 acres. It gets very expensive to explore for the small pipes. Various techniques have been tried to look for them. So far, they have been unsuccessful finding any to the south of Prairie Creek Pipe or the park.

U-Haul: Do experts think there's a chance to find commercial diamonds in Arkansas?

Mike Howard:There are still people spending money, exploring and working toward that end, yes.