Kimberlite
Kimberlite is a rare, ultramafic igneous rock famous as the primary source rock for natural diamonds. It originates from partial melting deep in Earth's mantle, often more than one hundred fifty kilometers down, and erupts violently through narrow, carrot-shaped volcanic pipes. Named after Kimberley, South Africa, where major diamond-bearing pipes were first mined, it is a chaotic mixture of olivine, mica, and fragments picked up from surrounding rock on its way to the surface.
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Kimberlite properties at a glance
| Type | Igneous Rocks |
|---|---|
| Mohs hardness | 5–6.5 |
| Luster | Dull to vitreous |
| Fracture | Uneven |
| Specific gravity | 2.5–2.9 |
| Transparency | Opaque |
| Common colors | Blue-gray ('blue ground'), Yellow ('yellow ground', weathered), Dark green, Gray |
| Rarity | common |
Geology & background
Kimberlite magma originates unusually deep in the mantle, within the diamond stability field, and its eruptions are thought to be extremely rapid, carrying deep mantle material, including diamonds and mantle xenoliths, to the surface fast enough that the diamonds do not have time to convert back into graphite. The resulting volcanic pipes, called diatremes, are typically small and carrot-shaped, filled with a breccia-like mixture of altered olivine, phlogopite mica, garnet, and ilmenite alongside chunks of country rock torn loose during ascent. Major kimberlite fields occur in South Africa, where the rock takes its name, as well as Russia's Siberian craton, Canada's Northwest Territories, and Botswana, all regions underlain by old, thick, stable continental crust called cratons.
Prospectors searching for kimberlite look for its distinctive indicator minerals, particularly pyrope garnet, chromite, and chrome diopside, which weather out of eroding pipes and concentrate downstream in soil and stream sediment.
How to identify Kimberlite
Kimberlite is typically a dark greenish-gray to bluish rock with an altered, weathered surface and a mixed texture of rounded olivine grains, mica flakes, and rock fragments in a fine-grained groundmass. Its luster varies by mineral, its streak runs gray to greenish, and its hardness is hard to pin down since it is a heterogeneous mixture, not a single mineral. Fresh kimberlite weathers fast to a soft clay called blue ground, so it is most reliably confirmed by its indicator minerals rather than appearance alone.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Kimberlite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Kimberlite worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Field identification
- Construction or aggregate when suitable
- Landscape interpretation
- Education
Interesting facts
- Kimberlite pipes are the primary natural source of diamonds, having carried them rapidly up from the mantle without letting them convert to graphite.
- The rock is named for Kimberley, South Africa, site of one of history's most famous diamond rushes.
- Prospectors often locate buried kimberlite pipes by tracing trails of eroded indicator minerals like pyrope garnet downstream.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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