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'Superdeep' Diamonds Formed From Seawater

The diamonds contain chemical information about what goes on deep inside the Earth. →

You might already know that the oceans absorb carbon dioxide from the atmosphere. But you might be surprised to discover that some of that carbon makes it to the sea floor and gets pushed into the Earth by the movement of the Earth's tectonic plates, where it comes in contact with the mantle and forms a rare type of diamond.

In a newly published article in the scientific journal Nature, researchers from Great Britain's Bristol University shed new light on the process by which the Earth's recycling of carbon through plate subduction, in which the edge of one plate slides under the edge of another plate, and how that process forms so-called superdeep diamonds.

The researchers, who experimented with small samples of synthetic ocean floor rock at high pressures and temperatures, found that those slabs released most of their carbon at conditions equivalent to depths of 186 to 435 miles below the Earth's surface.

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In addition, the researchers tested the reaction of the melted slabs with the mantle. They were able to reproduce the mineral makeup observed in superdeep diamonds, which generally form at depths of below 155 miles.

The research not only provides an explanation for how superdeep diamonds are formed, but shows that they essentially are a snapshot of the deepest portions of the Earth's carbon cycle. That makes them a useful tool for understanding what goes on inside our planet.

"One of the most amazing ideas that comes from this work is that superdeep diamonds are like marathon runners that have just crossed the finish line," Bristol scientist Andrew R. Thomson explained in a press release.

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"The difference in this case is the diamonds have just completed one of the most mind baffling journeys possible, from the ocean floor to around 700 km (435 miles) depth and back to the surface," he said. "Fortunately for scientists, their mineral inclusions are like stopwatches recording the entire journey, and with further work we will hopefully reveal many more remarkable secrets about their epic journey."

As Thomson's colleague Simon Kohn added: "Superdeep diamonds hold great potential for future research on the Earth's volatile cycles, and we now know much more about the fundamental process that forms them. We will be able to use the wealth of information that is trapped inside the diamonds to build a detailed picture of processes occurring hundreds of kilometers beneath our feet."

After diamonds form from the carbon in those slabs of seafloor, the rock slowly is pushed back toward the upper mantle. There, the heat of a plume of magma propels it to the surface in a jet of molten kimberlite, a volcanic rock that preserves the diamonds.

The diamonds contain chemical information about what goes on deep inside the Earth.

All gemstones are rare; some are just harder to find than others.

In fact, there is no consensus on what is the rarest mineral or the rarest gemstone because there is no consensus on the definition of "rarity," according to the Gemological Institute of America. However, many of the stones in this series come from only one or two localities in the entire world, so in that sense, they are scarce.

Pink Star Diamond

In the image above, model Annabeth Murphy-Thomas poses with the Pink Star diamond at Sotheby's auction house in central London. The diamond was put up for auction in Geneva on Nov. 13, 2013, at $60 million, an already record price for a gemstone, and sold for $83 million. Diamond cutter Isaac Wolf of New York purchased the Pink Star diamond ring, and renamed it the Pink Dream. The diamond measures 1.06 inches by 0.81 inches (2.69cm by 2.06cm).

This Blue Moon diamond, discovered in South Africa in January, 2014, weighs in at 12.03 carats, and is the largest cushion-shaped stone in that category to ever appear at auction. The Gemological Institute of America declared the Blue Moon to be “internally flawless.” It was purchased in November, 2015, by Hong Kong businessman Joseph Lau, who spent $48 million on it for his seven-year-old daughter, Josephine.

This giant rock is said to be the biggest diamond unearthed in more than century. Second in size only to the 3,106-carat Cullinan diamond found in South Africa in 1905, this chunk was found in Botswana.

Opal is Australia’s national gemstone, and black opal is the rarest and most valuable of its kind, at times selling at prices that rival the best diamonds. The stone must have a rich, black background, but base colors come in all shades of gray, which is why opinions vary on what is a "true" black opal. Found in the Lightning Ridge area in northwestern New South Wales, black opals are natural, solid stones that absorb scattered white light, giving it brilliant spectral colors.

The Gemological Institute of America (GIA) calls musgravite "a rarity among the rare... a particular gem on our research examination 'want list.'" A very close relative of another hard-to-find gemstone, taaffeite (and often misidentified as such), musgravite was first discovered in 1967 in the Musgrave Range of South Australia. Facet grade -- the baseline measurement of how clean cut a sellable stone must be -- for musgravite was not reported until 1993. As of 2005, there were only eight musgravite specimens in the world.

Discovered in 1951 in Mogok, Burma, painite was once considered the rarest mineral on Earth. For decades, only two crystals were known to exist. It didn't obtain official gemstone status until 1957 when the British Museum conducted X-ray analysis on a sample. In 1979, a third crystal was recovered by the GIA. Today, more than a thousand crystals and crystal fragments have been found. However, only a small percentage of the rough are suitable for sale. Painite is made up of aluminium, calcium, boron, zirconium and oxygen. It gets its orange-red to brownish-red color from trace amounts of iron.

Jeremejevite is an extremely rare, aluminium borate mineral. It was discovered in the late 19th century and named after Pavel V. Jeremejev, a Russian mineralogist and engineer. Until recently, the only two known localities for jeremejevite were Mt. Soktuj in the Transbaikal region of Russia and Cape Cross, Swakopmund, Namibia. Not much is known about jeremejevite. The color is typically aquamarine, but other records show the mineral can also be dark blue, pale yellow-brown or colorless.

Red diamonds, just like any other diamonds, are made of compressed carbon. However, the brilliant red color in these diamonds is formed from a structural defect in the crystal lattice structure, which is why they are the rarest of the colored diamond collection. Only a handful have ever received the grade of "Fancy Red," meaning that they are pure red with no modifying color. Most are sold at market for millions of dollars. The Argyle mine in Australia is the primary producer of pink and sometimes red diamonds.

This transparent, blue gem first turned up in 1962 and has been found scattered throughout northern Tanzania in Africa. Ranging in color from light blue to pure blue to dark violet-blue, the deepest hues are valued most. Made popular by jewerly giant Tiffany & Co. in 1968, Tanzanite has seen wild price fluctuations over the years. Tanzania's violent political, social and economic conditions have made it difficult at times to mine the mineral. However, the nation remains the gem's only known source.

Although "red emerald” is its snazzy marketing name, and it was originally called "bixbite," this mineral goes by the name "red beryl" today. The brilliant red-purple color is not a trick of the light. The stone's actual chemistry is distinctive and separate from other beryls. It is found along fractures in topaz rhyolites. The gem crystallizes when rhyolite-derived gases, vapors from heated groundwater, and preexisting minerals and volcanic glass in the rhyolite react all at once. There is only one known commercial production of gem-quality red beryl in the world: the Ruby Violet (or Red Beryl) mine in the Wah Wah Mountains of Beaver County, Utah.

Still one of the rarest gems known today, this pinkish mineral was named after the Poudrette family, owners and operators of a quarry near Mont Saint-Hilaire, Quebec, Canada, where poudretteite was first found. It was discovered as a few tiny crystals during the mid-1960s, but wasn't recognized as a new mineral until 1986. The first documented gem-quality specimen of poudretteite wasn't discovered until 2000, when it was found in Mogok, Burma. This remarkable, flawless 9.41 carat poudretteite gem from Burma is truly one-of-a-kind. It is considered to be one of the largest -- if not the largest -- faceted poudretteite in existence.