How Natural Diamonds Are Made

Ancient Beginnings

Abundant amounts of carbon naturally occurring the the Earth's mantle melt and during millions of years under extreme temperatures and pressure form diamonds.

The time required for diamond creation means that diamonds must remain underground for at least 990,000,000 years, and many have stayed below the surface for 3.2 billion years. This makes diamonds some of the oldest minerals found on Earth. Thus far, the oldest rocks--zircon found in Australia--date from 4.3 billion years ago. These zircons formed during the earliest era of the 4.54 billion-year-old Earth, and just 1 billion years later, the oldest diamonds formed. The oldest diamonds date from the time of the origins of organisms capable of life, between 2.7 and 3.5 billion years ago. These archeans bore little resemblance to modern plants and animals, but scientists considered them living as they used photosynthesis to produce their own food.

Diamonds form only when carbon in the mantle has a temperature between 900 and 1300 degrees C and pressure between 45 and 60 kilobars (kB). These pressures and temperatures occur at depths between 90 and 120 miles underground.

Journey to the Surface of the Earth

Diamonds must stay under the conditions in which they form until the time they reach the surface. Incomplete diamonds may dissolve or melt should the temperature or pressure increase too suddenly.

To reach the surface, diamonds take an express elevator operated by volcanic forces. These "elevators" are igneous rocks better known as kimberlite pipes. The kimberlite mineral inside the pipes picks up the diamonds from where they form in the Earth's mantle and the internal magma eruption brings the diamonds close to the surface. After cooling, the magma forms kimberlite mineral with diamonds inside it. The speeds reached by the diamonds carried in these pipes can reach 10 to 30 km per hour. This rate prevents the diamonds from reverting into graphite, an ordinary form of carbon found on the Earth's surface.

Diamond Mines

Diamond mines are usually located where there are kimberlite pipes, but some diamonds have been discovered in alluvial deposits in rivers. Alluvial deposits do not produce many diamonds, averaging one carat of diamonds per 100 tons of mud.

The alluvial deposits are formed from kimberlite pipes that eroded away over the years, leaving behind the diamonds, but little other evidence of the previous volcanic activity. Many of the mines in the United States produce diamonds from alluvial deposits that were once volcanic kimberlite pipes, these include mines in Wyoming, Arkansas and Colorado. Diamond mines in Canada near its borders with Alaska and Minnesota lead some to speculate that diamonds also exist in those states.

More productive diamond mines exist elsewhere in the world. Africa has the greatest number of diamond mines with seven in South Africa, four each in Angola and Botswana and three each in Sierra Leone and Nambia. In Limpopo, South Africa, the Venetia Diamond Mine produces 40 percent of the gemstone-quality diamonds found worldwide. Russia also has five diamond mines of its own.

References

• Diamonds and Diamond Simulants
• Diamond Formation
• Diamond Occurance