Are Diamonds Geologically Common?

Check out this answer from Consensus:

Diamonds are geologically more common than their economic rarity might imply. They occur in various geological settings, from kimberlite pipes to ophiolitic mantle peridotites, and form at different depths within the Earth’s mantle. While only a small percentage of kimberlites are economically viable, the widespread occurrence of diamonds in the mantle highlights their geological abundance. Further research into the formation and distribution of diamonds will continue to enhance our understanding of these fascinating gemstones.

Diamonds, often perceived as rare and precious, have fascinated humanity for centuries. However, their geological occurrence and distribution tell a more complex story. This article explores the geological commonality of diamonds, drawing on recent research findings to provide a comprehensive overview.

Geological Occurrence of Diamonds

Diamonds are primarily found in two types of geological settings: kimberlite pipes and ophiolitic mantle peridotites. Kimberlite pipes are volcanic structures that transport diamonds from the mantle to the Earth’s surface. These pipes are the primary source of economically viable diamond deposits8. On the other hand, ophiolitic mantle peridotites, which are sections of the Earth’s mantle thrust onto the continental crust, also contain diamonds, although these occurrences are less economically significant1.

Kimberlite Pipes

Kimberlite-hosted diamond deposits are the most well-known and economically important sources of diamonds. These deposits are found in various regions, including southern Africa, where they have been extensively studied and mined8. The formation of diamonds in kimberlite pipes involves complex geological processes, including the growth of diamonds in the mantle and their subsequent transport to the surface by kimberlite magmas. These processes have occurred multiple times over geological history, resulting in diamonds of varying ages and compositions8.

Ophiolitic Mantle Peridotites

Diamonds have also been documented in ophiolitic mantle peridotites and podiform chromitites from different orogenic belts. These diamonds are typically small and contain distinctive inclusions, such as coesite and Ni-Mn-Co alloys, indicating their formation under ultrahigh-pressure (UHP) conditions1. The presence of diamonds in these settings suggests that they are more widespread in the oceanic mantle than previously thought1.

Depth of Diamond Formation

Diamonds form at various depths within the Earth’s mantle, ranging from the lithosphere to the lower mantle. Super-deep diamonds (SDDs), which form at depths between ~300 and ~1000 km, represent the deepest direct samples from the Earth’s interior6. These diamonds provide valuable insights into the geological processes occurring at extreme depths, including the recycling of crustal materials into the mantle2 6.

Super-Deep Diamonds

Super-deep diamonds are rare, comprising only about 1% of the entire diamond population. However, they are scientifically significant as they offer a glimpse into the deep mantle’s conditions and processes. For instance, blue boron-bearing diamonds, which form in the lower mantle, indicate the presence of boron in diamond-forming fluids at great depths, suggesting a viable pathway for the deep-mantle recycling of crustal elements2.

Economic and Scientific Value

While diamonds are geologically more common than their economic deposits suggest, only a small fraction of discovered kimberlites are economically viable for mining8. The majority of diamonds in economic deposits are derived from the subcontinental lithospheric mantle, with harzburgite and eclogite being the primary source rocks5. These diamonds, often referred to as smooth-surfaced crystalline diamonds (SCD), have significant commercial value due to their size and quality5.

Are diamonds geologically common?

Antony Burnham has answered Unlikely

An expert from Australian National University in Geochemistry, Petrology, Mineralogy

It certainly used to be the case that a small number of companies had a near-monopoly on diamonds. This is less true nowadays with many large and smaller companies owning mines competing for market share. The term ‘limiting supply’ is a bit of an oversimplification, because obviously there is no point mining diamonds at a loss and so an economic judgement has to be made when considering any mining operations. New mines open every year, so it’s not as though there is a massive stockpile in a warehouse somewhere.

Diamonds are not common at the Earth’s surface (though it is a matter of active scientific debate how abundant they are in the Earth’s mantle). They are mostly found in volcanic rocks called kimberlites, with a more minor proportion in related volcanic rocks or in secondary sedimentary deposits that have formed by erosion of these primary deposits. Even the most diamond-rich primary deposits only have 1 – 2 grams of diamond per tonne of rock (5 – 10 carats, though each carat of rough diamond only equates to half a carat of polished gemstone), and often tens or hundreds of times less. You could walk around the pit of a diamond mine for an hour and not see a diamond. Only a small proportion of diamond deposits are profitable to mine, so it’s nowhere near the rarest mineral on Earth, but the majority of economic deposits are concentrated in a handful of countries.

As for how precious diamonds are, that is not a scientific question but a matter of individual taste and societal fashion.