Kimberlite is a dull gray-green to bluish rock with an uneven, pebbly texture that sets it apart from most other igneous rocks. At first glance, it can look unremarkable, almost like a dirty concrete, but a closer look reveals a mix of crystal sizes, colorful mineral grains, and chunks of foreign rock that make it one of the most visually distinctive rocks on Earth. It’s best known as the primary source of diamonds, and its appearance reflects the violent journey it took from deep in the Earth’s mantle to the surface.
Color and Overall Texture
Fresh, unweathered kimberlite typically ranges from dark gray-green to bluish gray. Miners in South Africa historically called this fresh material “blue ground.” The surface feels gritty and rough, and the rock is fine to medium grained in its base material, or groundmass. What makes kimberlite immediately recognizable is its uneven grain size. Scattered throughout that fine-grained groundmass are larger crystals and rock fragments of varying sizes, giving it a lumpy, porphyritic look. Imagine a gray-blue cement studded with pebbles and crystals of different colors, and you have a reasonable mental picture.
The two main types of kimberlite look slightly different up close. Group I kimberlite, which is the more common variety, has a cleaner groundmass without much visible mica. Group II kimberlite, sometimes called “micaceous kimberlite,” contains flakes of a bronze-colored mica (phlogopite) scattered through the groundmass, giving it a slightly glittery, warmer appearance.
How Weathering Changes the Color
Kimberlite doesn’t stay blue-gray for long once exposed to air and water. As it weathers, the rock transitions from blue ground to what miners call “yellow ground,” a softer, crumbly, yellowish-brown material. This color shift happens because iron-bearing minerals in the rock oxidize (essentially rust), and primary minerals break down, releasing calcium, magnesium, and iron that form new compounds at the surface. Microorganisms in the soil accelerate this process by producing acids that dissolve the original minerals faster.
If you find kimberlite at the surface, it will almost certainly be in some stage of this yellow-brown weathering. Only samples from deeper in a mine or freshly exposed outcrops will show the original blue-gray color. The weathered version is softer and crumbles more easily, which is partly why kimberlite pipes often form depressions or low spots in the landscape rather than ridges.
Crystals You Can See With Your Eyes
The most common visible crystal in kimberlite is olivine, a mineral that appears as rounded, glassy grains ranging from pale green to yellowish green. Olivine grains in kimberlite come in two size classes: larger grains (called macrocrysts) that range from about 0.5 to 10 millimeters across and tend to be rounded and worn-looking, and smaller grains under 0.5 millimeters that are more angular and well-shaped. In weathered samples, olivine often alters to a rusty brown or is replaced entirely by serpentine, a softer greenish mineral, which contributes to the rock’s overall greenish cast.
Beyond olivine, kimberlite contains several brilliantly colored minerals that prospectors use to identify it in the field:
- Pyrope garnet: These round, glassy grains range from deep red-purple and blood-red through lavender to orange-yellow. They’re notably clear and lack the dark inclusions common in other garnets. Many are coated in a dull grayish rind that, when scraped away, reveals a distinctive dimpled surface resembling orange peel.
- Chrome diopside: A bright emerald-green mineral that’s hard to miss. Grains tend to be blocky and rectangular with flat, smooth faces, making them look almost like tiny green tiles.
- Magnesian ilmenite: A glossy, metallic black mineral with a mirror-like sheen on fresh surfaces. It looks like polished obsidian but is much heavier in your hand due to its high iron and titanium content.
These three minerals are so characteristic of kimberlite that geologists call them “indicator minerals.” Finding even a few of them in stream sediments or soil is enough to trigger a more thorough search for a kimberlite source nearby.
Rock Fragments Trapped Inside
One of kimberlite’s most striking visual features is the chunks of foreign rock embedded in it, called xenoliths (literally “foreign stones”). These are pieces of the Earth’s mantle and crust that the kimberlite magma ripped loose and carried to the surface during eruption. They can range from pea-sized to boulder-sized.
The most common xenoliths are pieces of peridotite, a dense, coarse-grained rock from the upper mantle. Fresh peridotite xenoliths are pale green to olive colored, made mostly of olivine crystals large enough to see individually. In weathered kimberlite, these chunks often alter to a rusty brownish mass. Eclogite xenoliths are rarer but more visually striking: they consist of red garnet grains set in a green matrix of pyroxene, creating a bold red-and-green speckled appearance. In kimberlite samples, eclogite xenoliths are often heavily altered, with a mottled, patchy look where the original minerals have partially broken down.
These embedded fragments give kimberlite a chaotic, fragmental appearance that no other rock type quite matches. A single hand sample might contain a rounded green peridotite nodule the size of a golf ball, several red garnet grains, and a scattering of black ilmenite crystals, all held together in a blue-gray or greenish matrix.
How to Tell Kimberlite Apart From Similar Rocks
Kimberlite is sometimes confused with other dark, fine-grained igneous rocks like basalt or lamprophyre. A few features help separate it. First, the uneven grain size is key: basalt has a fairly uniform texture, while kimberlite is full of crystals and rock fragments of wildly different sizes. Second, the presence of bright red garnets, emerald-green chrome diopside, or glossy black ilmenite is a strong indicator, since these minerals are rare in most surface rocks. Third, kimberlite is softer than basalt and often feels slightly soapy or greasy to the touch due to serpentine formed from altered olivine.
In weathered form, kimberlite can look like ordinary clay-rich soil, which is why many kimberlite pipes went unnoticed for centuries. The yellowish, crumbly surface material gives little hint of the diamond-bearing rock below. Prospectors historically relied on finding indicator minerals in riverbeds rather than spotting kimberlite outcrops directly.