How To Identify Kimberlite (Tips For Beginners)

Kimberlite can be confused with other materials.

In this article, we’ll teach you about kimberlite and how to identify it.

How To Identify Kimberlite (Tips For Beginners)

What Is Kimberlite?

The material referred to as kimberlite, also called blue ground, is a dark-colored, often fragmented, intrusive rock that may contain diamonds in its rock matrix.

It is named after the town of Kimberley, South Africa, where in 1869 the discovery of an 83 carat diamond inside kimberlite rock spawned the world’s largest diamond rush.

Since that time, kimberlite has been the most important source of diamonds in the world.

They contain a variety of minerals with chemical make-up that indicate they were formed under high pressure and temperature within the mantle. 

Its most abundant minerals include olivine, garnet, serpentine, calcite, rutile, and magnetite.

Kimberlites are magmatic rocks that form deep in Earth’s interior and are brought to the surface by small but violent volcanic eruptions.

These volcanic explosions produce vertical columns of kimberlite rock that rise from deep magmatic reservoirs.

Their classical carrot shaped pipelike structures are important for delivering material – including diamonds from the asthenosphere- through the lithosphere and spewing on to the crust of earth’s surface.

Initially, the origin of diamonds was still unknown, and the precious gems were thought to have been found in alluvial deposits.

This is because of a phenomenon that occurred in South Africa whereby the tops of the two first known kimberlite pipes had been weathered down and the diamonds had been washed into the Vaal and Orange rivers, where the diamonds were first found.

Weathered kimberlite was colored yellow by limonite, and so was called “yellow ground”.

Deeper workings revealed less altered rock, serpentinized kimberlite, which miners named “blue ground”.

Kimberlite is known to erode relatively quickly.

About 6,400 kimberlite pipes have been discovered in the world.

They are generally only found in very old sections of the Earth’s crust.

Though Kimberlite can most often be found in Southwest Africa, this worldly rock is also indigenous to New York, Australia, Tanzania and various other parts of the world.

Deposits of kimberlite have also been found in the Prince Charles Mountains on the eastern side of Antarctica.

Although scientists agree that mining will not take place on that continent.

Tips For Identifying Kimberlite

Locating

Aside from old age rockhounding, geologists have also developed techniques to identifying sample from the sky using air magnetics.

Kimberlite is a peculiar igneous rock that also exhibits strong magnetism.

Due to the trace mineral, Magnetite, that is found inside this heavy, diverse looking rock, it has very distinctive magnetic and electromagnetic signatures.

As kimberlite pipes erode, the kimberlite and the surrounding rocks are attacked by weathering. 

Usually, the diamond pipes erode somewhat more rapidly than the surrounding rocks and form bowl-shaped depressions. 

As the rocks erode, the kimberlite weathers to a yellowish product referred to as yellow ground. 

Fresh kimberlite rock, called blue ground, holds up much more strongly and must be crushed or soaked in water with appropriate added salts to break it apart and extract pieces of the rock.

Characteristics

Unless extremely familiar with rocks and minerals, one could easily misinterpret kimberlite for any other surface stone.

Visually, there are two distinctive characteristics that separate kimberlite from other minerals.

The kimberlite matrix is made up of clay minerals, presented in blue, purple and yellowish-white colors.

It will usually have noticeably different colored specs or spots within and surrounding the stone.

These spots are crystals such as garnets and possibly diamonds.

Once in hand, kimberlite should have a porphyritic texture, with large, rounded crystals surrounded by a fine-grained ground mass.

It is a very heavy piece of rock in relation to size.

Kimberlites are a very dense and diverse stone, but generally have distinct trace mineralogy.

They are dominated by carbonate and significant amounts of forsterite olivine.

The most important signature indicator minerals are chromite, pyrope, Mg-rich ilmenite, and olivine.

How Kimberlite Can Be Confused for Other Specimens

Like Kimberlite, Lamproite is an ultrapotassic mantle-derived volcanic or subvolcanic rock.

Kimberlites & lamproites are unusual igneous bodies, both have unique pipe-shaped geometries.

Their mode of early formation is only moderately understood because they form so deeply in the earth, they have never been observed forming.

Kimberlites & lamproites are both known from scattered localities throughout the world, and both can be diamondiferous, or diamond bearing.

Although the two rocks hold many similarities, they have significant differences to help tell them apart.

Kimberlites are made in the low-heat flow regions, whereas lamproites occur in the high-heat regions.

Kimberlites and lamproites were formed in different times; in particular, the most productive kimberlitic formation processes were observed to have occurred some 400 million years ago.

Kimberlite and lamproite bodies also differ in morphology: lamproites compose small subvolcanic bodies with lava flows, while kimberlites form volcanic pipes with no lavas.

Kimberlites contain highly silica-undersaturated minerals, while ultrabasic lamproites—silica-undersaturated ones, priderite and wadeite, the characteristic accessory minerals of lamproites, are not observed in kimberlites.

The primary melts of kimberlites and lamproites were derived from different types of mantles as well.

Summary

Kimberlites are very mineralogically distinctive.

If visual identification fails you, it will be easy for a professional to determine what your rock is, if sent out for a thin section.

Avoiding misidentification is key to a successful rock hounding or rock collecting experience.

It is important to identify if you have a kimberlite verse most other sedimentary surface rocks because, with kimberlite, chances are much higher you could discover something much more beautiful and valuable inside!

how to identify kimberlite