The largest pink diamond unearthed in 300 years has just come out of Lulo alluvial mine in Angola.

The diamond, called the Lulo Rose, weighs in at 170 carats. As impressive as that sounds, the Lulo Rose is just one of more than 25 diamonds of over 100 carats to be recovered from Lulo since operations began in 2015. Angola, located in southwestern Africa, is the world’s fourth largest producer of diamonds, just ahead of South Africa. While South Africa produces a greater volume of diamonds, Angola’s are typically larger and more valuable. So what is it about Angola that produces such enormous diamonds?

How Diamonds Are Formed

To answer that question, let’s first look at how diamonds are formed. Geologists are still learning the specifics of diamond formation, but we do know that diamonds typically form in a rock called kimberlite, and there’s a good chance that where there’s kimberlite, there are diamonds. Good, but not a sure thing: of the 7000 kimberlites that have been studied over the past 150 years, about 14% contained diamonds, and less than 1% are sufficiently diamond-rich to be worth mining. Still, locating kimberlite is your best bet for locating diamonds.

Kimberlite is the most common host matrix for diamond because of the depth at which it forms. While most gemstones form deep in the crust, diamonds grow even deeper, in the upper mantle (the only other gemstone that forms in the upper mantle is peridot), particularly on parts of the upper mantle that come in contact with extremely ancient and stable rock formations called cratons. This is where heat and pressure are sufficient to melt carbon and recrystallize the 2D carbon molecule into a 3D crystal matrix. Carbon atoms crystalize into diamond in molten kimberlite, and the kimberlite is then thrust upward forcefully by jets of magma. It is that upward ejection that gives kimberlite its distinctive pipe or carrot shape.

These kimberlite pipes have massive diameters. They are the reason why diamond mines take the form of huge concentric circles dug into the earth. Sometimes, whether through volcanic activity or erosion, diamonds are exposed to the surface and carried away by rivers to be deposited in nearby sediment. Such was the fate of the Lulo Rose, discovered in an alluvial mine.

The force with which kimberlite is shot out of the mantle is truly incredible due to the mix of volatile CO2 and the extreme pressure at those depths. Sometimes, it is great enough to distort the shape of the diamond’s crystal lattice. It is this distortion that gives pink diamonds like the Lulo Rose its color. In most cases, gemstone coloration is the result of elements trapped in a gemstone’s crystal lattice. Chromium or vanadium makes an emerald green, iron and titanium make a sapphire blue, and traces of nitrogen tint a diamond yellow. But pink diamonds get their color not from trace elements but from distortions in the crystal lattice. The great stress that leads to this distortion hints at the volatile geological history of Angola.

Why Does Angola Produce Such Large Diamonds?

To get a sense for the geological history of Angola, one need only look at a map. See how Africa and South America fit together like puzzle pieces? 180 million years ago, the Atlantic Ocean didn’t exist, and the two continents formed a supercontinent called Gondwana. Had the countries (or any countries) existed at that time, when dinosaurs ruled the earth, Angola would have bordered Brazil. But Gondwana split into two continents, and now a slowly widening ocean sits between them.

While most kimberlite was formed in the Archean Eon, 2 to 4 billion years ago, Angola’s kimberlite was likely formed between 1.5 billion and 600 million years ago. But it wasn’t until about 100 million years ago that the Gondwana supercontinent began to buckle under its own weight. It is likely that the immense strain of the rifting that created the Atlantic Ocean created heat and pressure great enough to recrystallize carbon in magmatic kimberlite into large diamond crystals, and to eject that kimberlite upwards into the areas of northeastern Angola where it is found today. Lupaca sits on an ancient oceanic transform boundary, a location where two tectonic plates slide past each other.

This unique geology is responsible for some of the most spectacular diamond finds of the 21st century. In 2017, miners at the Lulo site unearthed a 227 ct. diamond. And back in 2008, Lonrho Mining’s Lulo concession turned up a 131.5 ct. diamond. But it was in 2016 that the largest Angolan diamond of any type was mined in Lulo, the 404.2 ct. 4 de Fevereiro diamond. It was eventually cut into a 163.41 ct. emerald cut that sold for $22 million at Christie’s.

While not as big as 4 de Fevereiro, the Angola Star, mined in 2007, may be the most beautiful of the large diamonds to come out of Angola. The 217 ct. type IIA diamond was set in a necklace in its rough form, the large polyhedral crystal held in place by an eight-pronged claw covered in pave diamonds. Set in a piece of jewelry in its natural state, it becomes an otherworldly, almost mystical object, though what could be more of this world than a chunk of pure carbon produced by the hidden tempest churning beneath our feet?

The Lulo Rose is just the latest in a long line of enormous diamonds discovered in Angola. While the Lulo Rose is the largest pink diamond found in Angola so far, chances are it won’t hold that record forever. There are hundreds of kimberlite structures in northeastern Angola, but there is still a lot of untapped potential in the region.