Kimberlite
Potassic ultramafic igneous rock (olivine-rich)
A rare ultramafic volcanic rock that erupts from deep in the mantle and is the primary natural source of diamonds.
- Mohs hardness
- variable, ~3-6 (matrix soft, contains hard minerals)
- Color
- bluish-grey (blue ground) to yellowish when weathered
- Type
- igneous
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Overview
Kimberlite is a rare, volatile-rich ultramafic igneous rock best known as the principal natural source of diamonds. It is dominated by olivine, with minerals such as phlogopite mica, garnet, ilmenite, and chromite, and often contains fragments of mantle and crustal rock carried up during eruption.
Kimberlite occurs in pipe-shaped intrusions called diatremes or kimberlite pipes. Fresh, less weathered rock is called "blue ground," while weathered near-surface kimberlite is "yellow ground."
Not all kimberlites contain diamonds, but those that do are mined intensively, as in South Africa, Russia, Canada, and Botswana.
Formation & geology
Kimberlite forms from gas-rich, ultramafic magma that originates at great depths in the Earth's mantle, often more than 150 km down, within the stability field of diamond. Charged with carbon dioxide and water, this magma rises rapidly through deep fractures and erupts explosively, blasting carrot-shaped pipes (diatremes) toward the surface.
The rapid ascent is crucial: it carries diamonds (and mantle xenoliths) up quickly enough that the diamonds do not revert to graphite. Kimberlites are confined to ancient, stable continental cratons. Classic occurrences include Kimberley, South Africa, which gave the rock its name.
How to identify it
Identify kimberlite by its dark, often bluish-grey (blue ground) or yellowish weathered (yellow ground) appearance and its brecciated texture containing rounded mantle xenoliths and crystals of olivine, garnet (pyrope), and ilmenite. The matrix is relatively soft and alters easily.
Indicator minerals such as deep-red pyrope garnet, green chrome diopside, and picroilmenite are key clues used by diamond prospectors.
Its pipe-like field occurrence on ancient cratons and abundant mantle fragments distinguish it from ordinary basalt or peridotite. Hardness varies because the altered matrix is soft but it carries hard crystals.
Uses & significance
Kimberlite's overwhelming economic importance is as the primary ore of natural diamonds, mined for both gem and industrial stones. Major diamond mines worldwide exploit kimberlite pipes.
Beyond diamonds, kimberlite and its xenoliths are scientifically invaluable, providing direct samples of the deep mantle and information about Earth's interior, age, and the conditions of diamond formation.
Indicator minerals from kimberlite guide diamond exploration. The rock itself has little ornamental or building use.
Frequently asked questions
Why is kimberlite important?
It is the main natural source of diamonds, which it carries up from the deep mantle during rapid, explosive eruptions.
What are blue ground and yellow ground?
Blue ground is fresh, unweathered kimberlite, while yellow ground is the oxidized, weathered near-surface zone.
Does all kimberlite contain diamonds?
No. Only some kimberlite pipes are diamond-bearing, and even those usually contain very low concentrations of diamonds.
Where does kimberlite form?
It rises from depths over 150 km in the mantle and erupts only through ancient, stable continental cratons.
Kimberlite guides
In-depth guides for identifying, valuing, and understanding Kimberlite.
Kimberlite identified by the community
Real specimens identified with Rock Identifier.
