How are natural diamonds made?

Diamonds are dazzling gemstones that captivate our hearts and have a remarkable story to tell. Imagine them as revealing the secrets from the depths of our planet. Unlike other gemstones like ruby, sapphire, and emerald which form in the Earth’s crust, diamonds are formed deep within the Earth’s mantle. These sparkling diamonds are the Earth’s treasures that bring forth their beauty.

Contrary to popular belief, diamonds don’t magically transform from coal. Instead, they form under intense pressure and heat, but not from direct conversion of solid carbon. The traditional “graphite to diamond” story isn’t the whole truth. So, how do diamonds form in the mantle?

How diamonds are formed in the Earth's mantle

Two key ingredients are essential. Carbon must be present in a mantle fluid or melt in sufficient quantity, and the melt or fluid must become reduced enough so that oxygen does not combine with carbon. Natural diamonds typically form 150–200 kilometers below the Earth’s surface. This process occurs selectively beneath the oldest continents, known as cratons, which have remained stable for billions of years. Cratons possess thick continental roots with favorable temperature profiles conducive to diamond formation. Geologists refer to this region as the “continental lithospheric mantle.”

There are two types of diamonds formed. First, lithospheric diamonds form within the continental roots of cratons. They are carried upward to the surface by rare volcanic eruptions called kimberlites. Second, superdeep diamonds form much deeper in the Earth, in regions of the mantle known as the transition zone (410 to 660 km) and the lower mantle (>660 km).

Diamonds are brought up nearer to the surface of the Earth by movements in the Earth's mantle. After formation, superdeep diamonds are transported to shallower depths within the mantle, likely through mantle convection cells. Eventually, they are brought to the surface by subsequent kimberlite eruptions. Mantle convection, driven by plate tectonics, plays a crucial role by providing the necessary fluids for diamond formation, and also facilitates the return of diamonds to the surface. Additionally, mantle convection generates the kimberlites, volcanic rocks that deliver diamonds to the Earth’s surface.

Luxury diamond ring

But different diamonds form in various environments and through diverse mechanisms. Your diamond ring or jewellery most likely have different origins from your friend's or family member's diamond jewellery.

In summary, diamonds aren’t just pretty; they’re Earth’s storytellers, revealing secrets from the depths. So next time you admire a diamond, remember: it’s more than a gem—it’s a beautiful treasure for you and your loved ones!

Proposal for marriage with diamond ring

Sources: GIA, Cambridge.