While gem enthusiasts worldwide enjoy the thrill of hunting for precious stones close to the Earth’s surface, some gemstones originate from astonishing depths, challenging the boundaries of Earth’s mantle. The deepest of these coveted gems is the diamond, renowned for its beauty, industrial utility, and invaluable scientific insights.
Diamonds’ formation remains a topic of ongoing scientific exploration, but one aspect is clear: they crystallize under extreme pressures. Most natural gemstones trace their origins to the upper mantle, a region lying between 93 to 186 miles (150 to 300 kilometers) beneath the Earth’s surface, where pressures soar beyond 20,000 atmospheres.
For years, diamonds vied with a gem known as peridot for the title of the deepest-occurring gemstone. Peridot, the gem form of olivine, a mineral constituting more than half of the upper mantle, extends from the Earth’s crust’s base down to approximately 255 miles (410 kilometers). However, in 2016, scientists unveiled a collection of superdeep diamonds, retrieved from depths around 410 miles (660 kilometers), and in 2021, another batch was traced back to a remarkable depth of 466 miles (750 kilometers).
These groundbreaking discoveries effectively settled the debate regarding the deepest gemstone. Researchers drew these conclusions by examining the diamonds’ crystallization patterns and their inclusions, which are traces of minerals or fluids trapped within the gems during their formation. The presence of minerals like bridgmanite and iron-nickel-carbon-sulfur melt in these superdeep diamonds pointed to their formation in the lower mantle, comprising approximately 75% bridgmanite, surrounded by liquid metal and methane. At such depths, pressures can reach a staggering 235,000 atmospheres.
Diamonds are not only exceptionally deep in their origin but are also believed to be exceedingly ancient. Some estimates suggest that diamonds on the Earth’s surface today may have formed up to 3.5 billion years ago, although many are considerably younger. Their extraordinary durability stems from the robust chemical bonds holding them together. Diamonds consist of carbon and, because they form under extreme pressure, require immense force to break their bonds. Exposing a diamond to temperatures exceeding 1,652 degrees Fahrenheit (900 degrees Celsius) leads to its degradation into graphite.
Remarkably, these profound insights into diamonds’ origins and properties were not obtained through subterranean exploration. Even the deepest man-made dig, the Kola Superdeep Borehole in Russia, only scratched the Earth’s surface, reaching a modest depth of 7.8 miles (12.6 kilometers). Instead, diamonds find their way to the surface through a unique type of magma called kimberlite. Kimberlite magmas are highly volatile, erupting at speeds exceeding 100 feet per second (30 meters per second) and extracting diamonds from the surrounding rocks as they ascend. This natural phenomenon catapults gemstones that formed over billions of years to the Earth’s surface within months, if not hours.
Beyond their aesthetic appeal and natural hardness, which makes them indispensable to industries employing them in blades, drill bits, and polishing powders, diamonds hold priceless scientific information. Ananya Mallik, an experimental petrologist at the University of Arizona, underscores the significance of diamonds as invaluable sources for understanding the Earth’s interior and its processes. Researchers have employed these gemstones to unravel the Earth’s geological history, revealing that early Earth was less tectonically active than it is today. Moreover, carbon signatures found in diamonds suggest that the carbon cycle extends deep into the Earth’s interior. Recent diamond studies have even disclosed the presence of water at greater depths within the mantle than previously anticipated and unveiled entirely new minerals.
In the world of gemstones, diamonds not only shine with incomparable allure but also illuminate the depths of Earth’s mysteries, combining their intrinsic beauty with unmatched scientific value.
