Diamonds are forged under geological conditions far beneath the Earth’s surface. Their creation requires immense pressure (45 to 60 kilobars) and high temperatures (900 to 1,300 degrees Celsius). These conditions are met only within the Earth’s mantle, at depths between 150 and 250 kilometers. Rare, violent volcanic eruptions bring these crystals to the surface, carrying the diamonds within specific types of igneous rock.
Kimberlite: The Main Source Rock
Kimberlite is an ultramafic igneous rock and the most significant host for diamonds, accounting for 85 to 90% of all natural diamond production. This rock originates at depths of 150 to 450 kilometers within the mantle beneath ancient continental plates known as cratons. The magma is rich in volatiles like carbon dioxide and water, which fuel the explosive ascent toward the surface.
This rapid eruption prevents the diamonds from transforming back into graphite, which occurs under lower-pressure conditions. As the magma breaches the crust, it forms characteristic vertical structures known as kimberlite pipes or diatremes. These pipe formations preserve the deep-earth material within a cone-shaped structure. The rock is a hybrid mixture, often containing high levels of olivine and other mantle minerals, indicating its deep origin.
Lamproite: The Secondary Source Rock
Lamproite is a source rock that is much less common than kimberlite, contributing about 5 to 10% of global diamond output. Like kimberlite, lamproite magma forms deep within the mantle, at depths exceeding 150 kilometers. Its chemical composition is distinctly different, characterized as ultrapotassic, meaning it is richer in potassium.
The mineralogy of lamproite includes potassium-rich minerals like titanium-phlogopite and potassium-richterite. Lamproite eruptions also create volcanic pipes, but these are often wider and shallower than kimberlite pipes. The Argyle mine in Australia was a notable example of a lamproite deposit, famous for producing high volumes of diamonds, including the rare pink variety.
Alluvial Deposits: Diamonds Separated from the Source
Many diamonds are found in secondary locations called alluvial deposits, rather than their original host rock. These deposits form after millions of years of weathering and erosion break down the primary kimberlite and lamproite pipes. Because diamonds are the hardest and highly dense natural substance, they resist this breakdown process.
The liberated diamonds are transported by water, carried downstream by ancient and modern river systems. They eventually settle in low-energy environments, such as riverbeds, floodplains, deltas, coastlines, and the ocean floor, forming placer deposits. This natural sorting process is significant because weaker or flawed diamonds often shatter during the journey. Consequently, alluvial deposits frequently yield a higher proportion of durable, high-quality gem-grade stones. Historically, these secondary sources were the first places where diamonds were discovered and mined.