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Learn · Lesson 4

How and Where Natural Diamonds Form

HOK-DIA-LESSON-00004Monitored

Learning Objectives

Core

High pressure alone is not enough to form a natural diamond. Diamond growth requires an appropriate combination of pressure and temperature, a suitable chemical environment, a carbon source, and a medium and reaction pathway that permit the crystal to grow and survive.

A large share of studied natural diamonds belongs to lithospheric populations associated with the deep, cool roots of ancient continents. As a practical orientation, the Book gives roughly 150–200 km and about 900–1300 °C for many such populations, while preserving the real variation among deposits and diamond populations. Peridotitic and eclogitic parageneses are two major environmental groupings.

Rarer sublithospheric or superdeep diamonds carry information from substantially greater depths.

Apply

A natural diamond is best understood as the result of several distinct stages. First comes growth in the deep mantle, often within old, cool cratonic roots. A very long interval of storage may follow. Much later, kimberlitic or lamproitic magma may capture the crystal and carry it toward the surface. The age of the diamond and the age of the rock that transported it are therefore not the same datum.

Lithospheric populations are commonly associated with peridotitic and eclogitic environments. Mineral inclusions, their chemistry, and geothermobarometric models can help reconstruct conditions, but a single visual feature is not enough to establish mineral identity or an exact depth. Inclusions must also be interpreted according to whether they formed before, during, or after diamond growth.

Many diamonds crystallize from mobile carbon-bearing fluids or melts. Growth need not be one continuous event: a crystal can record multiple generations, and the same deep system may favor growth during one stage and dissolution during another. Fibrous diamonds and microinclusions are especially informative archives of such media.

Superdeep diamonds provide a different window below the continental lithosphere. High-pressure phases and their retrograde products may support an interpretation involving the mantle transition zone or lower mantle, but the strength of that inference depends on how well the original phase is preserved and how uniquely it can be reconstructed.

For practical interpretation, keep these stages separate: growth → storage → transport → resorption → exposure/erosion → discovery. Each stage can leave its own evidence, and evidence from one stage should not automatically be assigned to another.

Check Your Understanding

Before accepting a geological conclusion, ask:

Conclusion Boundary

Escalate when morphology, an inclusion, or geological context is being used to infer a precise mineral identity, age, pressure-temperature condition, depth, or geographic origin. Those conclusions require appropriate analytical methods and an explicit treatment of uncertainty, not visual similarity alone.

Common Mistakes

“Diamond forms in kimberlite.”
For most natural diamonds, kimberlite is a transporter, not the primary growth environment.

“If we identify the mineral inclusion, we know the mine.”
No. Mineralogy can inform a geological model, but it is not automatically a geographic identifier.

“A three-billion-year-old diamond grew for three billion years.”
No. It may have spent most of that time stored stably in the mantle.

“Superdeep diamonds show that all diamonds form at the same depth.”
No. Natural diamonds belong to different geological populations.

Remember

Natural diamond is best read as a geological record of growth → preservation → capture → transport → surface exposure, with distinct evidence and limitations at each stage.

Sources

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