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How Diamonds Work
by Kevin Bonsor
On your next trip to the local shopping mall, stop by one of the jewelry stores. Notice the diamond jewelry that takes up the majority of the showcase, and the number of people hovering over the counters trying to pick out a diamond for their loved one. There will surely be a salesperson explaining the "4 Cs" -- cut, clarity, carat and color -- to a young shopper, and explaining why one diamond is better than the one right next to it. Why all the fuss over diamonds?
Photo courtesy Smithsonian Institution/Chip Clark
The Hooker Diamond necklace, earrings and ring on display at the National Museum of Natural History
Diamonds are just carbon in its most concentrated form. That's it -- carbon, the element that makes up 18 percent of the weight of your body. In many countries, including the United States and Japan, there is no other gemstone as cherished as the diamond, but in truth, diamonds are no more rare than many other precious gems. They continue to demand higher market prices because the majority of the diamond market is controlled by a single entity.
In this article, we will track a diamond from the time it is formed to when it reaches the Earth's surface. We will also examine the artificial rarity created by the diamond cartel, De Beers, and briefly discuss the properties of these gems.
The Origin of Diamonds
Carbon is one of the most common elements in the world, and is one of the four essentials for the existence of life. Humans are more than 18-percent carbon. The air we breathe contains traces of carbon. When occurring in nature, carbon exists in three basic forms:
Archean Cratons
Temperatures can reach 1,652 F (900 C) in Archean cratons. These are common places for diamonds to form. Archean cratons are stable, horizontal geological formations created billions of years ago that have been unaffected by major tectonic events, according to Rex Diamond Mining Corp. These cratons are found in the center of most of the seven continents (most tectonic activity takes place around the edges).
- Diamond - an extremely hard, clear crystal
- Graphite - A soft, black mineral made of pure carbon. The molecular structure is not as compact as diamond's, which makes it weaker than diamond.
- Fullerite - A mineral made of perfectly spherical molecules consisting of exactly 60 carbon atoms. This allotrope was discovered in 1990.
Diamonds form about 100 miles (161 km) below the Earth's surface, in the molten rock of the Earth's mantle, which provides the right amounts of pressure and heat to transform carbon into a diamond. In order for a diamond to be created, carbon must be placed under at least 435,113 pounds per square inch (psi or 30 kilobars) of pressure at a temperature of at least 752 degrees Fahrenheit (400 Celsius). If conditions drop below either of these two points, graphite will be created. At depths of 93 miles (150 km) or more, pressure builds to about 725,189 psi (50 kilobars) and heat can exceed 2,192 F (1,200 C).
Most diamonds that we see today were formed millions (if not billions) of years ago. Powerful magma eruptions brought the diamonds to the surface, creating kimberlite pipes. On the next page, you'll find out about these pipes.
Space Diamonds
Diamonds are not exclusive to Earth. Scientists believe that diamonds may one day be found on the moon. Samples of rock brought back from the moon indicate that carbon is 10 times more abundant in the Earth's crust than the moon's, according to the Artemis Project, a group whose goal is to establish a permanent moon community. But this group believes that there may be diamonds under the moon's surface that Apollo astronauts were unable to detect. There is also some scientific evidence that diamonds may be found in larger abundance on Neptune and Uranus. Neptune and Uranus contain a lot of the hydrocarbon gas methane. Researchers at the University of California, Berkeley, have shown that focusing a laser beam on pressurized liquid methane can produce diamond dust. Neptune and Uranus contain about 10-percent to 15-percent methane under an outer atmosphere of hydrogen and helium. Scientists think that this methane could possibly turn to diamond at fairly shallow depths.
Kimberlite Pipes
Kimberlite is named after Kimberly, South Africa, where these pipes were first found. Most of these eruptions occurred between 1,100 million and 20 million years ago.
Kimberlite pipes are created as magma flows through deep fractures in the Earth. The magma inside the kimberlite pipes acts like an elevator, pushing the diamonds and other rocks and minerals through the mantle and crust in just a few hours. These eruptions were short, but many times more powerful than volcanic eruptions that happen today. The magma in these eruptions originated at depths three times deeper than the magma source for volcanoes like Mount St. Helens, according to the American Museum of Natural History.
The magma eventually cooled inside these kimberlite pipes, leaving behind conical veins of kimberlite rock that contain diamonds. Kimberlite is a bluish rock that diamond miners look for when seeking out new diamond deposits. The surface area of diamond-bearing kimberlite pipes ranges from 2 to 146 hectares (5 to 361 acres).
Diamonds may also be found in river beds, which are called alluvial diamond sites. These are diamonds that originate in kimberlite pipes, but get moved by geological activity. Glaciers and water can also move diamonds thousands of miles from their original location. Today, most diamonds are found in Australia, Borneo, Brazil, Russia and several African countries, including South Africa and Zaire.
Diamonds are found as rough stones and must be processed to create a sparkling gem that is ready for purchase. In the next section, you will learn how a diamond is cut and polished.
Diamond Properties
As mentioned before, diamonds are the crystallized form of carbon created under extreme heat and pressure. It's this same process that makes diamonds the hardest mineral we know of. A diamond ranks a 10 on the Mohs Hardness Scale (see below). It can be anywhere from 10 to hundreds of times harder than a mineral ranked nine on the Mohs scale, such as corundum. Corundum is a class of minerals that includes rubies and sapphires.
It is the molecular structure of diamonds that makes them so hard. Diamonds are made of carbon atoms linked together in a lattice structure. Each carbon atom shares electrons with four other carbon atoms, forming a tetrahedral unit. This tetrahedral bonding of five carbons forms an incredibly strong molecule. Graphite, another form of carbon, isn't as strong as diamond because the carbon atoms in graphite link together in rings, where each atom is only linked to one other atom.
Diamonds are cut along planes of weaknesses to create the shape and facets of the
diamonds we see in the jewelry store.
Cutting Diamonds
There are special techniques that are used to cut and shape a diamond before it gets to the jewelry store. Diamond cutting creates the facets that you see in the diagram above. Diamond cutters use these four basic techniques:
- Cleaving - In order to remove any impurities or irregularities in the diamond, a rough diamond is placed in quick-drying cement. A sharp groove is then carved into the diamond, using another diamond or a laser, along planes of weakness. Then, a steel blade is placed in the groove and a sharp blow to the blade splits the stone. It is then removed from the cement.
- Sawing - Sometimes, diamonds have to be cut against a cleavage plane, which cannot be done with cleaving. Using a phosphor-bronze blade rotating at about 15,000 rpm, the saw slowly cuts through the diamond. Lasers are also being used to saw diamonds.
- Bruting - The diamond is placed in a lathe, and another diamond in the lathe is rubbed against it to create the rough finish of the girdle, the outside rim of the diamond at the point of largest diameter.
- Polishing - To give the diamond its finished look, it is placed onto the arm above a rotating polishing wheel. The wheel is coated with diamond powder that smoothes the diamond as it is pressed against the wheel.
Rating Diamonds
Diamonds are judged on several factors that determine their beauty. Most diamonds never reach the consumer market because they are too flawed. Often, these diamonds are used for industrial purposes -- as an abrasive, for drill bits or for cutting diamonds and other gems. If you've ever purchased a diamond, you've heard of the "4 Cs:"
- Cut - This refers to how the diamond has been cut and its geometric proportions. When a diamond is cut, facets are created and the diamond's finished shape is determined.
- Clarity - This is the measurement of a diamond's flaws, or inclusions that are seen in the diamond. Clarity levels begin with Flawless and move down to Very Very Slight (VVS), Very Slight (VS) and Slightly Included (SI).
- Carat - This is the weight of a diamond. One carat is equal to about 200 milligrams.
- Color - In referring to transparent diamonds, the color scale runs from D to Z, beginning with Icy White -- the color of the most expensive diamonds -- and ending with a light yellow.
Other unique qualities of the diamond include its transparency, luster and dispersion of light. A diamond that is created from 100-percent carbon will be completely transparent. Diamonds often contain other elements that can affect the color. Although we often think of diamonds as being clear, there are also blue, red, black, pale green, pink and violet diamonds. These colored diamonds are the truly rare ones.
The Mohs Scale
The Mohs Scale is used to determine the hardness of solids, especially minerals. It is named after the German mineralogist Friedrich Mohs. The scale reads as follows, from softest to hardest:
- Talc - easily scratched by the fingernail
- Gypsum - just scratched by the fingernail
- Calcite - scratches and is scratched by a copper coin
- Fluorite - not scratched by a copper coin and does not scratch glass
- Apatite - just scratches glass and is easily scratched by a knife
- Orthoclase - easily scratches glass and is just scratched by a file
- Quartz (amethyst, citrine, tiger's-eye, aventurine) - not scratched by a file
- Topaz - scratched only by corundum and diamond
- Corundum (sapphires and rubies) - scratched only by diamond
- Diamond - scratched only by another diamond
Source: Kingzett's Chemical Encyc.
A Perception of Rarity
Diamonds are made in the molten magma deep inside the Earth. Only nature can create diamonds, but it is people who have created the artificial rarity that has spurred demand for these gems. Carbon is one of the most common elements in the world, and diamonds are a form of carbon. Naturally occurring diamonds are no more rare than many other precious gems. The truly rare transparent diamonds are those rated as flawless, meaning that they don't have the slightest imperfection.
Photo courtesy Smithsonian Institution Some diamonds, like the Hope Diamond (pictured here), are extremely rare. However, most diamonds are much more abundant than we are led to believe.
Marketing Diamonds
Prior to the 1930s, diamond rings were rarely given as engagement rings. Opals, rubies, sapphires and turquoise were deemed much more exotic gems to give as tokens of one's love, according to the book "Twenty Ads that Shook the World," by James B. Twitchell. Twitchell goes on to describe how De Beers changed the world diamond market.
This idea of connecting diamonds to romance was captured in a brilliant ad campaign begun in the 1940s, causing demand for diamonds to increase. Surely you've heard the De Beers advertisement telling you that "A Diamond is Forever." This ad campaign, which was created by the N.W. Ayer advertising agency in 1947, changed the diamond market. In 2000, Advertising Age magazine named the ad campaign the slogan of the 20th century. De Beers infiltrated Japan with the same ad campaign in the 1960s, and the Japanese public bought into the idea as much as the Americans did.
Later ads by De Beers told consumers to hold onto their family's diamond jewelry and to cherish them as heirlooms -- and it worked. This eliminated the aftermarket for diamonds, which further enabled De Beers to control the market. Without people selling their diamonds back to jewelers or to other people, the demand for new diamonds increased.
There are fewer than 200 people or companies authorized to buy rough diamonds from De Beers. These people are called sightholders, and they purchase the diamonds through the Central Selling Organization (CSO), a subsidiary of De Beers that markets about 70 percent to 80 percent of the world's diamonds. De Beers sells a parcel of rough diamonds to a sightholder, who in turn sends the diamonds to cutting facilities and then to distributors.
There are rough diamonds sold outside the CSO. These diamonds come from small producers in Australia, Russia and some African countries. The cost of these diamonds is still largely influenced by the prices set by the CSO.
Diamonds are the most coveted of all precious gems, as is witnessed by the extremely high demand for them. While this has not always been the case, diamonds are nonetheless exquisite gems that go through a long, tedious refining process from the time they are pulled from the ground to when you see them in the jewelry store. And, while some of the mystique of diamonds may be gone -- they're just carbon, after all -- the diamond will likely continue to be a highly coveted jewel, because, well, "A Diamond is Forever."
Famous Diamonds
- The Cullinan - This is the largest diamond ever found. This 3,106-carat diamond was found in 1905 in Transvaal, South Africa. In 1907, the diamond was presented to King Edward VII of England. Later, the diamond was cut into nine major stones, including the 530.20-carat Star of Africa diamond that is set in the Royal Scepter displayed in the Tower of London.
- The Hope Diamond - Possibly the most famous diamond in America, this 45.52-carat diamond is on display at the National Museum of Natural History in Washington D.C. Its history dates back to the 1600s, when it was originally a 112.1875-carat diamond. In 1668, it was purchased by King Louis XIV, of France. It is believed to have been originally found in the Kollur mine in Golconda, India. The diamond was recut in 1673, creating a smaller 67.125-carat stone.
- The Excelsior - Perhaps the second-largest diamond ever found, the Excelsior was found in 1893 in South Africa. The original stone weighed about 995 carats. In 1904, I.J. Asscher and Company of Amsterdam cut the Excelsior into 21 polished stones weighing between 1 and 70 carats.
- The Great Mogul - Believed to be the third-largest uncut diamond ever found, it was discovered around 1650. Its original size is said to have been 787.50 carats, but it was cut to just 280 carats. The diamond is named for Shah Jehan, who built the Taj Mahal. After the diamond was cut, he fired the cutter for doing such a poor job. Mysteriously, the whereabouts of the Great Mogul diamond are unknown today.