A diamond is a unique geological structure composed of pure carbon atoms tightly bonded in a crystalline lattice. This simple chemical composition and rigid atomic arrangement make the diamond the hardest natural material known. Formation requires conditions of extreme pressure and temperature found deep within the Earth’s interior. Diamonds are ancient time capsules, offering geologists a direct link to the planet’s earliest history.
The Primary Age of Terrestrial Diamonds
The majority of diamonds recovered from the Earth’s crust are incredibly old, predating complex life. These stones typically date back to the Precambrian Eon, ranging from approximately 1 billion to 3.5 billion years old. This vast timeframe encompasses the Archean and Proterozoic eras, representing a substantial portion of Earth’s 4.54-billion-year history. The oldest diamond-forming events occurred within cratons, the most stable, ancient sections of continental crust.
The kimberlite and lamproite volcanic eruptions that transport these diamonds to the surface are much younger geological events. The diamonds are xenocrysts—foreign crystals—that are billions of years older than the surrounding rock that carried them up. This age disparity highlights the diamond’s role as a preserved relic, stored deep within the mantle until its rapid ascent.
The Geological Process of Diamond Formation
Diamond creation demands a precise combination of high pressure and high temperature (HPHT), existing within the Earth’s mantle at depths of 150 to 250 kilometers. This region, beneath the oldest continental blocks, is known as the lithospheric mantle keel. Temperatures here hover between 900 and 1,300 degrees Celsius, and the intense pressure forces carbon atoms into the compact structure of diamond instead of graphite.
The carbon source is varied, including primordial carbon and subducted oceanic crust carrying organic material into the mantle. Diamonds remain stable at these depths until a rare, deep-source volcanic event occurs. Eruptions forming kimberlite and lamproite pipes are the only natural mechanism that safely transports diamonds to the surface. The magma must travel extremely quickly to prevent the diamonds from converting back into graphite as pressure decreases nearer the surface.
Determining Diamond Age Through Scientific Analysis
Because a diamond is essentially pure carbon, it lacks the radioactive elements necessary for direct dating using standard methods. Geologists instead rely on tiny mineral impurities, known as inclusions, trapped within the diamond’s crystal structure during formation. These microscopic inclusions, which can be silicates, sulfides, or carbonates, are sealed off from the surrounding mantle and act as time capsules.
Scientists apply radiometric dating techniques to these inclusions to determine a crystallization timeline. The Rhenium-Osmium (Re-Os) decay system is frequently used on sulfide inclusions, where Rhenium-187 decays into Osmium-187 over billions of years. The Samarium-Neodymium (Sm-Nd) system is applied to silicate inclusions like garnet. By measuring the precise ratio of the parent isotope to the stable daughter isotope, researchers calculate the exact age when the inclusion and the host diamond crystallized.
Variations in Diamond Age and Origin
While most commercial diamonds are ancient lithospheric diamonds, not all gems share the same origin or age. A distinct category is the “super-deep” diamond, which forms much farther down, potentially in the transition zone between the upper and lower mantle, at depths between 360 and 800 kilometers. Some super-deep diamonds have been dated to a younger age range, between 450 million and 650 million years old.
Super-Deep Diamonds
These diamonds are often large and pure, sometimes containing unique inclusions that point to an environment rich in metallic iron and less oxygenated than the shallower mantle.
Impact Diamonds
A smaller number of diamonds form more recently and closer to the surface due to meteoric impact events. The massive shock pressure momentarily creates the necessary conditions for diamond formation. These impact diamonds, along with those found in metamorphic rock settings, demonstrate that diamond creation is not limited to a single time or place.