What Rocks Contain Diamonds

Rocks containing diamonds are geological formations where natural diamonds, Earth's hardest known material, originate. Kimberlite and lamproite are the primary rocks that contain diamonds.

Diamonds are created under extreme conditions deep in the planet's upper mantle, typically between 150 to 250 kilometers below the surface. Here, temperatures exceed 1,000°C, which is hotter than the melting point of most metals, and pressures surpass 45 to 60 kilobars (4.5 to 6 gigapascals). These conditions allow carbon atoms to crystallize into the diamond structure. These precious gems remain concealed until volcanic eruptions bring them closer to the surface.

Rocks Contain Diamonds

The rocks that host diamonds, such as kimberlite and lamproite, are not only fascinating for their diamond content but also because they serve as geological time capsules. They provide a glimpse into the ancient conditions of the Earth's deep interior, guiding geologists in their search for these valuable crystals. Below, we explore the types of rocks associated with diamonds:

Types of rocks that contain diamonds, including kimberlite and lamproite.

Kimberlite

Kimberlite is not only the rock most commonly associated with diamonds but also the cornerstone of the vast majority of commercial diamond mining activities worldwide.

What Is Kimberlite

Kimberlite is an ultramafic igneous rock, born from the fiery depths of the Earth's mantle. It forms when molten rock or magma, rich in volatiles like water and carbon dioxide, ascends rapidly and erupts with explosive force. This eruption creates unique vertical structures known as kimberlite pipes or diatremes. These pipes serve as conduits, bringing diamonds, along with other mantle fragments, swiftly to the surface.

Key Characteristics

Texture: Kimberlite often exhibits a porphyritic texture, where large crystals, or phenocrysts, are set within a finer-grained matrix. This reflects the complex cooling process from the magma's rapid ascent.

Color:

• Fresh Kimberlite ("Blue Ground"): Can be blue or green, attributed to the presence of fresh olivine and other minerals.
• Weathered Kimberlite ("Yellow Ground"): Turns yellowish due to the oxidation of iron-bearing minerals.

Kimberlite rock samples with porphyritic texture in green and blue colors.

Minerals: Kimberlite contains:

• Olivine: A common indicator of its mantle origin.
• Mica (Phlogopite): Often seen as shiny flakes within the rock.
• Pyroxene: Adds to the rock's dark color.
• Garnet: Particularly Pyrope garnet, which is a strong indicator of diamond presence.
• Indicator Minerals: Such as chromite, ilmenite, and diamond inclusion minerals like eclogitic garnet, which geologists use to trace back to diamond-rich sources.

Diamond Content: While not all kimberlite bodies are diamondiferous, those that are can be exceptionally rich, making kimberlite the primary source of diamonds on a global scale.

Kimberlite Locations

• Kimberley, South Africa: Home to the Big Hole, one of the earliest and most famous diamond mines.
• Mirny, Russia: Features the Mir Mine, known for its vast open pit.
• Ekati and Diavik Mines, Canada: Located in the Northwest Territories, these are among the northernmost diamond mines in the world.

Kimberlite exploration involves searching for these indicator minerals in surface samples or through geophysical methods, leveraging the rock's unique signature to pinpoint potential diamond deposits.

Rough, sparkling natural diamonds embedded within a dark, igneous rock matrix known as kimberlite.

Economic Importance

Famous diamond mines like the Big Hole in South Africa and Canada’s Ekati and Diavik mines are built around kimberlite pipes. Exploration techniques include searching for indicator minerals and using geophysical surveys to detect magnetic anomalies caused by kimberlite.

Lamproite

Though less common than kimberlite, lamproite is another volcanic rock capable of hosting diamonds. Its unique mineral composition and volcanic origins make it a noteworthy player in diamond mining.

What Is Lamproite

Lamproite forms from magma rich in potassium and magnesium, originating deep within the Earth's mantle. Like kimberlite, it erupts explosively, creating pipes or dykes that occasionally carry diamonds to the surface. This explosive ascent from depths where diamond formation conditions exist can bring these gems to shallower crustal levels, where they might be mined.

Lamproite rock samples from the Jurassic period of Sierra Leone and the Blue Ball Lamproite Dike of western Arkansas.

Key Characteristics

Texture: Can vary, often vesicular (containing gas bubbles) due to the rapid cooling of magma and gas release during eruption.

Color: Typically greenish, brownish, or black, reflecting its mineralogical makeup.

• Fresh Lamproite: Can be quite dark, often appearing black or dark green due to its mafic (rich in magnesium and iron) composition.
• Weathered Lamproite: Over time, it can take on a more yellow or brown appearance due to the iron oxidation process.

Minerals: Lamproite Contains rare minerals like:

• Leucite, a potassium-rich feldspathoid, which is less common in other rock types.
• Olivine, which might be altered to serpentine upon weathering.
• Clinopyroxene such as diopside or augite.

Diamond Content: Some lamproites are significant sources of diamonds. The Argyle Mine in Australia, for instance, is renowned for its lamproite-hosted diamonds, particularly the rare pink diamonds, which are among the rarest and most valuable in the world due to their unique color caused by distortions in the crystal lattice.

Yellow diamond nestled in the sandy, grainy lamproite tuff contrasts with the vivid pink diamond, a rare gem from the Argyle mine's lamproite deposit.

Lamproite Locations

Argyle, Western Australia: Perhaps the most famous lamproite diamond mine, Argyle is renowned for its high-quality diamonds, including the rare pink diamonds. It has been a major source of diamonds since its discovery in 1979, though it ceased mining operations in November 2020.

Ellendale, Western Australia: The Ellendale Diamond Project consists of several lamproite pipes. Although the focus has shifted away from diamond mining due to economic factors, it's historically significant for its diamond production.

Crater of Diamonds State Park, Arkansas, USA: This site is unique because it's one of the few places where the public can search for diamonds. The diamonds here are found within a lamproite intrusion known as the Prairie Creek diatreme.

Leucite Hills, Wyoming, USA: While not primarily known for diamonds, this area is famous for its lamproite outcrops and has been studied for its potential to host diamonds.

• Wyomingite is an ultrapotassic, ultramafic igneous rock known as a diopside-leucite phlogopite lamproite, which is characterized by its specific mineral composition including diopside, leucite , and phlogopite, along with lesser amounts of apatite, calcite, magnetite, and sometimes olivine. Wyomingite itself does not typically contain diamonds.

Eclogite and Peridotite

While they might not have the fame of kimberlite or lamproite, eclogite and peridotite are significant mantle rocks that can occasionally contain diamonds, playing a crucial role in our understanding of diamond formation.

What Are They

Eclogite: This rock is characterized by its high density and typically greenish hue, primarily made up of:

• Garnet: Often red or pink, indicating high-pressure conditions.
• Omphacite: A type of pyroxene notable for its jade-like appearance.

Eclogite, a metamorphic rock, and peridotite, an igneous rock, side by side for comparison.

Peridotite: Dominated by:

• Olivine: Which gives the rock its characteristic green color when fresh.
• Pyroxene: Contributing to the rock's often dark appearance.

Formation and Transport

Both eclogite and peridotite are formed deep within the Earth's mantle where diamonds are created under extreme conditions:

Diamond Formation: Within these rocks, carbon crystallizes into diamond due to the immense pressures (over 50 kilobars) and temperatures (around 1,000°C or more) found at depths where these rocks naturally occur.

Transport to the Surface

Eclogite: Can be brought up through orogenic (mountain-building) processes where subducted oceanic crust is pushed into the mantle, then later exhumed or brought up by tectonic uplift.

Peridotite: Sometimes reaches the surface through processes like serpentinization (hydration of olivine into serpentine) which can make the rock less dense and more buoyant, or through volcanic activity when pieces of the mantle are incorporated into magmas that erupt.

Diamonds within these rocks can either be primary inclusions, formed as the rock itself, or xenocrysts, where they have been transported from another part of the mantle or crust. The presence of diamonds in eclogite or peridotite can provide clues about the deep Earth's processes, making these rocks not only potential sources of diamonds but also valuable for geological research.

Placer Deposits

Diamonds are not exclusively found in primary igneous sources such as kimberlite and lamproite pipes. Through weathering and erosion, diamonds can be liberated from these primary sources and transported to form secondary, alluvial, or placer deposits. These placer deposits represent a significant source of diamonds, where geological processes concentrate the diamonds eroded from primary sources, often through fluvial transport and deposition.

What Are Placer Deposits

Placer deposits are accumulations of valuable minerals, including diamonds, that have been eroded away from their primary rock and concentrated by natural sorting processes in sedimentary environments. 

A diagram illustrating the formation and distribution of diamonds in placer deposits, where diamonds are eroded from their primary source sources like kimberlite pipes, and transportation by rivers, and their final concentration in secondary placer deposits.

Key Characteristics

Location: Typically found in:

• Riverbeds, where flowing water sorts heavier minerals to the bottom.
• Beaches or coastal sands, where wave action concentrates diamonds.
• Alluvial fans and other depositional landscapes where water or glaciers once flowed.

Mineral Association with Placer Deposits

Diamonds in placer deposits are often found alongside other heavy minerals like:

• Gold: Due to its density, gold also concentrates in similar environments.
• Garnet: Especially the heavier varieties, which are resistant to weathering.
• Magnetite: A common iron oxide that, like diamonds, sinks to lower layers due to its weight.

Placer Deposits Diamond Characteristics

Rounded Shape: Due to the mechanical abrasion during transport, diamonds in placer deposits tend to be rounded or even flattened, losing their original octahedral or dodecahedral forms.

Diamond Placer Deposits Locations

The beaches of Namibia: Famous for marine diamond mining, where diamonds are found along the coast due to the erosion and transport by the Orange River from inland kimberlite deposits.

Riverbeds in Brazil and India: Historically significant for diamond placer mining, where rivers have carried diamonds away from their igneous sources, depositing them in alluvial sediments.

Placer mining for diamonds involves techniques like dredging, panning, or sluicing, where large volumes of sediment are processed to extract the diamonds. These deposits are critical not just for diamond production but also because they provide insights into the geological history of diamond distribution and the erosive processes shaping Earth's surface.

The Kenenday mine site in Guinea, where workers sift through river sediments to extract alluvial diamonds.

Why Diamonds Form in Certain Rocks

Depth and Pressure: Diamonds crystallize under conditions of extreme pressure and temperature found in the Earth's mantle, specifically within the diamond stability field. This zone lies approximately 140–200 kilometers below the Earth's surface where the pressure exceeds 50 kilobars and temperatures reach above 1,000°C.

Rapid Transport: The unique characteristic of kimberlite and lamproite magmas is their ability to ascend from the mantle to the surface at high speeds. This rapid transport is critical for diamond preservation because it prevents the diamonds from transforming back into graphite, which is the more stable form of carbon at lower pressures.

Volatile Magmas: Both kimberlite and lamproite are rich in volatile components like carbon dioxide (CO₂) and water (H₂O). These volatiles facilitate explosive eruptions, allowing the magma to shoot up through the crust, bringing diamonds with it. The high volatile content helps in generating the necessary pressure for such rapid ascent.

How Diamonds Are Found in These Rocks

Exploration

• Geological Indicators: Geologists search for specific minerals like garnet, chromite, ilmenite, and chrome diopside, which are often found with diamonds in kimberlite or lamproite.
• Geophysical Methods: Magnetic surveys are instrumental in detecting kimberlite pipes or lamproite deposits due to their unique magnetic signatures.
• Sediment Analysis: Analyzing stream or glacial sediments for these indicator minerals can pinpoint areas where diamond-bearing rocks might be.

Mining

Primary Deposits (Kimberlite and Lamproite):

• Open-Pit Mining: Used for near-surface deposits to extract large volumes of rock.
• Underground Mining: Employed when deposits are deeper or when surface mining becomes less viable.

Secondary Deposits (Placer)

• Dredging: A method for extracting diamonds from riverbeds, beaches, or the ocean floor.
• Panning: A simple, traditional method for separating diamonds from lighter sediment in alluvial deposits.

Identification: Diamond Indicators in Rocks

The presence of certain minerals in rocks like kimberlite or lamproite often signals the potential for diamond deposits. Here's what to look for:

Associated Minerals: Key indicators include garnet (especially pyrope), chrome diopside, ilmenite, and spinel. These minerals share a deep mantle origin with diamonds, making them significant markers for diamond exploration.

Texture and Inclusions: Diamond-bearing rocks might reveal small inclusions of other minerals like olivine or garnet. These inclusions are not only telltale signs of a diamond's presence but can also affect the diamond's clarity and value in the gem trade.

Diamonds themselves are distinguished by their unique physical properties:

• Hardness: Diamonds are renowned for their hardness, scoring a 10 on the Mohs scale, making them the hardest natural material.
• Luster: They possess an adamantine luster, giving them a brilliant, highly reflective appearance.
• Color and Transparency: Diamonds can be entirely colorless or display a range of colors, from yellows and blues to pinks, each adding to their allure.

When identifying diamonds in their natural state, both the surrounding minerals and the diamond's own properties are crucial for prospectors and gemologists.

The Geological Legacy of Diamondiferous Rocks

The path from diamond formation in the mantle to their eventual discovery at the Earth's surface is as fascinating as the diamonds themselves:

Explosive Origins: Kimberlite eruptions are notably violent, creating vertical structures known as pipes. This explosive ascent preserves the diamonds by quickly removing them from the mantle's high-pressure environment.

Erosion and Transport: Over geological time, these primary deposits can be eroded by natural forces like rivers, leading to the formation of placer deposits. Here, diamonds are sorted and concentrated in environments like riverbeds or beaches, far from their original source.

Cultural and Economic Significance: Beyond their economic value, diamonds are embedded in our culture, often symbolizing eternity, love, or achievement. Their rarity and the geological processes required for their formation add to their allure.

Scientific Insights: Diamondiferous rocks are not just sources of diamonds; they are invaluable for understanding Earth's ancient history, mantle composition, and the dynamic geological processes that shape our planet. Each rock type, from the depth-born kimberlite to the sediment-bound placer diamonds, tells a unique story of Earth's inner workings.

In essence, thejourneyofa diamond from the depths of the Earth to being admired or utilized by humans is a testament to the geological marvels of our planet. These rocks, through their association with diamonds, continue to fascinate and drive both scientific inquiry and economic endeavors.

Read also:
How Are Diamonds Formed
What Types of Rocks Contain Gold
Do Diamonds Really Come From Coal