Genesis: A High-Pressure Journey from the Deep Earth

Diamonds are not formed in the Earth’s crust; they are direct samples from the deep mantle. Unlike minerals that require common crustal processes, diamonds only form under extreme conditions of pressure (4.5 to 6 GPa) and temperature (900 to 1300 °C). This very specific environment exists in the sub-continental lithospheric mantle, known to geologists as the Diamond Stability Zone.

For a diamond to survive, it needs a fast trip to the surface. If the carbon atoms spend too much time ascending through the “Graphite Stability Zone,” they will rearrange into common graphite. The only known transport mechanism that can bypass this zone fast enough is a Kimberlite or Lamproite eruption.

 Earth Cross-Section Figure – Show the 150-200 km Formation Depth)

The Kimberlite Elevator: A Supersonic Journey

What is a Kimberlite? It is a rare, volatile-rich (high CO₂ and H₂O), ultrabasic volcanic rock that originates from great depths (sometimes over 400 km). A Kimberlite pipe is essentially a supersonic volcanic elevator; it blasts through the crust at speeds approaching Mach velocity. This extreme speed causes the diamond and other mantle minerals to “quench,” locking their high-pressure structure in place before they can turn into graphite. In the field, we identify the carrot-shaped volcanic structure that has eroded over millions of years.

Diamond in Kimberlite-Diamond mine

Exploration: Chasing the Indicator Minerals

Geologists rarely find diamonds directly on the surface. Instead, we hunt for Indicator Minerals that were transported alongside the diamonds from the mantle. In the field, finding:

G9/G10 Garnets (Cr-pyrope with a distinct purple/lilac hue),

Magnesium-rich Ilmenite,

Chrome Diopside (Brilliant emerald green), …means you are likely within the vicinity of a kimberlite pipe.

The Liar’s Check: Don’t get Fooled

In the field, quartz or topaz can sometimes look like a rough diamond to an untrained eye. The simplest test is Hardness. Quartz (7) and Topaz (8) will not scratch Corundum (9). Diamond (10) will scratch everything. If you find a crystal with an isometric (cubic) form and adamantine lustre, and it passes the hardness test, you’ve found a true mantle traveler.

Industrial Value: Core Drilling and Cutting

While 20% of mined diamonds are gem-quality, the remaining 80% (known as Bort) are critical for the mining industry. Their extreme hardness makes them the ultimate material for drill bits, allowing exploration geologists to core through the toughest rock formations.