How Diamonds Form

English: Diamond Engagement Ring with side sto...
Diamond Engagement Ring with side stones – 1791 Diamonds (Wikipedia)

(A version of this post was first published at Helium on August 7, 2011.)

Have you ever wondered how the stone(s) in a diamond ring formed?

According to geologist Scott Johnson, “Contrary to popular culture, diamonds are not formed from the metamorphosis of coal under tremendous heat and pressure. It makes for nice poetry, but it’s not true.”

Natural Diamonds

Diamonds start out extreme and stay that way.

Your stone began its journey long ago when little crystals of carbon in Earth’s upper mantle, some 80-110 miles (125-175 km) underground, began to collect on the under-surface of a continent,  somewhat like barnacles accumulating on a ship’s keel. (Update, July 17, 2018: According to a new study, the deep roots of continents, about 100 miles below the surface, may be 1% to 2% diamond!)

It took at least a billion years for each diamond crystal to grow in the intense heat and pressure that happened as plate tectonics slowly moved the continent around.  Perhaps some crystals picked up trace elements along the way – say, the boron that makes the Hope Diamond blue – or little bubbles of gas that jewelers in your local shopping mall would eventually call flaws.

Then some mantle rock melted, moving upwards from great depths.  This is basically how all volcanoes form, but in our case, a strong chemical reaction happened between the rising magma and the continent above it – sort of like what happens when you mix vinegar and baking soda.

You know how that goes.  The result:  a supersonic kimberlite blast through the continent in a matter of hours, bringing the diamonds along with it up to the surface.

(Kansas Geological Survey)
Kansas Geological Survey

Luckily for us, such eruptions aren’t happening today.  All known kimberlite formations date back at least 70 million years and are often older than that.

Some rare diamonds – like the famous black diamonds, or carbonados, that are only found in Brazil and at one place in Africa – are older than the Earth and believed to have formed in space, coming in on an asteroid.

Indeed, asteroid impacts themselves can form diamonds on Earth, as the world learned when Russia recently announced the discovery of “trillions of carats” of diamonds under an impact crater.  Such “shock” diamonds are at best small and usually just industrial grade.

Most of the world’s gem quality diamonds come from kimberlite.

Synthetic Diamonds

A rose-cut synthetic diamond created by Apollo...
A rose-cut synthetic diamond created by Apollo Diamond using the CVD process. (Wikipedia)

Cubic zirconia and moissanite are different minerals that resemble diamonds, but people have been making real diamonds since the 1950s.

Researchers mimicked Nature by first  making diamonds using extreme heat and pressure.  Today, they grow a diamond this way in about four days.   These high-pressure, high-temperature (HPHT) lab diamonds are colored  and small.

An alternative process involves chemical vapor deposition (CVD) similar to the manufacture of  semiconductors.  The diamond maker pumps a gas containing carbon into a chamber where it condenses onto a solid surface.  Colorless diamonds about five times as big as their HPHT counterparts have been made this way.

Are synthetic diamonds as good as natural diamonds?  Yes, they are.

In 2007, the Gemological Institute of America began grading synthetic diamonds.  In March 2012, the Institute announced (PDF) that “the quality of CVD-grown diamonds had improved significantly in the decade since they were introduced. . . . [S]eparating such stones from natural diamonds is increasingly difficult and now requires advanced spectroscopic techniques because of their high clarity and color.”  (Of note, gemologists are different from you and me – when they describe a diamond’s “color,” they are actually talking about how colorless it is.)

Consumer Choice

Prices vary, but synthetic diamonds can cost up to 15% less than the same natural stone.  However, for some, price is not the issue – it’s great to wear something that’s not only beautiful but also billions of years old.  There are more choices available today than ever before.  Now that you know how diamonds form, go check some out!


  1. Nice summary about kimberlites! Though the origin of carbonados is still a mystery and also new “microdiamonds” were discovered in some granulite rocks which for sure could never reach depths and pressures thought to be necessary for diamond formation. There are obviously more ways to create diamonds, some more detail at

    And btw. that about price is not true anymore. China advanced a lot in synthesis of larger gem quality diamonds, including colored fancy diamonds. Actually, the risk of buying fake is now extremely high as these synthetic diamonds are cheaper and cheaper and very hard to recognize from natural ones 😦

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