In Virgin Valley, Nevada, you can spend an afternoon digging for rare black fire opals, while in Coalinga, California, visitors can sift through the dirt for pieces of the state’s official gemstone, benitoite. At the Crater of Diamonds State Park in Arkansas, aspiring gemhounds can pay as little as $10 to hunt for some of the world’s most coveted stones.
Each of these excursions requires little more than hand tools, yet most gems are found between 3 and 25 miles (5 and 40 kilometres) underground, and some extend much deeper.
But which gems are found deepest, and how do they make their way to the surface?
According to Lee Groat, a mineralogist at the University of British Columbia, the deepest known gemstones are diamonds, sought after for their beauty, industrial use and the scientific data they contain.
How diamonds are formed is still not fully understood, but laboratory experiments show that the gemstones only crystallise under extreme pressure. Most naturally occurring stones have been found in the upper mantle, at depths between 93 and 186 miles (150 and 300 km), where pressures can exceed 20,000 atmospheres.
This has long put diamonds in competition with a gem called peridot for the title of deepest occurring gemstone. Peridot is the gem form of a mineral called olivine, which makes up more than half of the upper mantle, which extends from the base of the crust down to 255 miles (410 km). But in 2016, scientists described a collection of super-deep diamonds from around 410 miles (660 km), and another batch in 2021 was determined to have come from a depth of 466 miles (750 km).
“It was hard to say whether diamonds or peridot were the deepest, but this pretty much settled the debate,” Groat said.
To come up with these estimates, the researchers looked at the diamonds’ crystallisation patterns as well as their inclusions – bits of minerals or liquids that are trapped inside the gemstones from their formation. Inclusions of a mineral called bridgmanite and iron-nickel-carbon-sulphur melt told the scientists that these super-deep diamonds probably formed in the lower mantle, which is about 75% bridgmanite, and that they grew from liquid metal surrounded by methane. At these depths, pressures can exceed 235,000 atmospheres.
Diamonds are also thought to be extremely old. Some estimates suggest that diamonds on the surface today may have formed up to 3.5 billion years ago, although many are much younger. Their longevity is attributed to the strength of their chemical bonds, says Groat. Diamonds are made of carbon, and because they form under pressure, “it takes a lot of force to break their bonds”. Heating a diamond above 1,652 degrees Fahrenheit (900 degrees Celsius) causes it to decompose into graphite.
Gemologists didn’t have to dig into the Earth to learn this; the deepest we’ve ever penetrated the planet’s interior, at the Kola Superdeep Borehole in Russia, barely scratched the surface at a depth of just 7.8 miles (12.6 km).
Instead, the diamonds are brought to the surface by a unique type of magma called kimberlite. Kimberlite magmas tend to be volatile, erupting at speeds of more than 100 feet per second (30 metres per second) and pulling diamonds from the surrounding rocks as they go. In this way, gems that have taken billions of years to form are hurled to the surface in months or even hours.
Beyond their aesthetic value and natural hardness, which make them attractive to industries interested in blades, drill bits and polishing powders, diamonds contain priceless scientific information, said Ananya Mallik, an experimental petrologist at the University of Arizona. In many cases, “diamonds are the only sources that researchers have for understanding the makeup of the planet’s interior and the processes that take place there,” she told Live Science.
By studying these gems, scientists have learned that the early Earth wasn’t as tectonically active as it is today. Other analyses have revealed carbon signatures consistent with photosynthesis, showing that the carbon cycle extends deep into the planet’s interior. More recently, scientists studying rare diamonds have found evidence of water deeper in the mantle than previously thought, and even discovered entirely new minerals.
“Diamonds have their own value because of their beauty, but their scientific importance makes them even more valuable,” Mallik said.
