Diamonds are prized globally for their brilliance, strength, and remarkable age. Chemically, they are crystalline structures composed entirely of pure carbon. While a common perception holds that all natural diamonds are billions of years old, the geological record reveals a much wider, and surprisingly recent, timeline for their formation. This challenges the idea that diamond creation is solely an ancient process, raising the question of how recently Earth has forged its hardest material deep within its mantle.
The Vast Age Spectrum of Natural Diamonds
The age range of natural diamonds spans nearly the entire history of the Earth. The oldest known diamonds, referred to as cratonic diamonds, crystallized between 2.5 and 3.5 billion years ago. These ancient crystals are found beneath the stable, thick sections of continental crust known as cratons, where they were stored in the deep mantle.
These extremely old diamonds represent the primary population brought to the surface by kimberlite eruptions. In contrast, scientific analysis has identified a distinct, much younger population tied to modern plate tectonics. The youngest diamonds found so far are estimated to be as recent as 60 to 90 million years old.
This relatively young age places their formation during the Cretaceous period, just prior to the extinction of the dinosaurs. The existence of these younger stones confirms that the intense conditions necessary for diamond crystallization have persisted into the more recent geologic past.
The Deep Earth Origins of Diamond Formation
Diamond formation requires extreme heat and crushing pressure found exclusively in the Earth’s mantle, primarily at depths exceeding 150 kilometers. Temperatures must reach between 900 and 1,300 degrees Celsius, and pressure must range from 4.5 to 6 gigapascals. These conditions force carbon atoms into the dense, tightly bonded crystalline lattice that defines diamond.
Two primary geological settings account for the world’s diamond supply, each with a different carbon source and age profile. Ancient diamonds formed deep beneath stable cratons, drawing carbon from the primordial mantle. This environment allowed the carbon to crystallize and remain stable for billions of years within the continental keel.
Younger diamonds are often linked to subduction zones, where one tectonic plate slides beneath another. This process carries carbon-rich materials from the surface, such as oceanic crust and sediments, deep into the mantle. Fluids released from this recycled material act as a solvent, transporting carbon atoms to the diamond stability field where they crystallize. This recycling mechanism explains how diamond formation is an ongoing process in tectonically active regions.
Scientific Methods for Dating Diamonds
Determining the absolute age of a diamond is complex because pure carbon lacks the radioactive elements needed for direct dating. Geologists instead rely on tiny, syngenetic mineral fragments trapped inside the diamond during its formation. These inclusions, such as garnet, zircon, and sulfide minerals, act as protected time capsules containing trace amounts of radioactive isotopes.
Scientists use radiometric dating techniques on these trapped inclusions to determine the diamond’s age.
Uranium-Lead (U-Pb) Dating
This common method is applied to inclusions like zircon or garnet that incorporate uranium into their structure. Uranium slowly decays into lead at a known rate, allowing researchers to calculate the time elapsed since the inclusion crystallized inside the diamond.
Rhenium-Osmium (Re-Os) Dating
This effective technique is useful for sulfide inclusions. It tracks the decay of Rhenium-187 into Osmium-187, a decay system with an extremely long half-life. By analyzing the parent-daughter isotope ratios within these protected inclusions, scientists can accurately pinpoint the moment the diamond crystallized deep within the Earth.