Diamond

Description

Diamond is a solid form of carbon with atoms arranged in a crystal structure called diamond cubic. It is the hardest known natural mineral and has the highest thermal conductivity of any known material. In its rough state, diamond typically appears as anhedral to euhedral crystals, often with rounded edges due to resorption during ascent. Common crystal habits include octahedra, dodecahedra, and cubes, or combinations thereof. Colors can range from colorless to yellow, brown, green, blue, pink, red, and black, depending on impurities and structural defects. Rough diamonds often have a greasy or adamantine luster and may exhibit growth features such as trigons (triangular etch pits) on octahedral faces.

How to Identify

Color

Rough diamonds can be colorless, yellow, brown, gray, or rarely blue, green, pink, red, or black. The most common colors are yellow and brown due to nitrogen impurities.

Luster

Adamantine to greasy. Unpolished surfaces may appear dull or greasy due to surface coatings or etching.

Texture

Typically smooth to slightly rough on crystal faces, often with characteristic growth features like trigons (triangular pits) on octahedral faces, or striations on dodecahedral faces. Resorbed diamonds may have a frosted or etched appearance.

Crystal Form

Most commonly octahedral, dodecahedral, or cubic. Combinations of these forms are also frequent. Macles (twinned crystals) are common. Irregular, rounded, or amorphous shapes (bort, carbonado) also occur.

Cleavage

Perfect octahedral cleavage in four directions {111}. This means it can be split along these planes, though it is difficult to initiate due to its hardness.

Geological Environment

Primarily found in kimberlite and lamproite pipes (volcanic conduits) that originate in the Earth's mantle. Also found in alluvial (placer) deposits, which are secondary deposits formed by the erosion and transport of primary kimberlite/lamproite sources. Rarely found in ultra-high-pressure metamorphic rocks or impact structures.

Key Facts

• Hardness: 10 (Mohs scale), the hardest known natural mineral.
• Specific Gravity: 3.50 - 3.53 g/cm³ (for pure diamond). Can vary slightly with impurities.
• Crystal System: Cubic (isometric)
• Color: Colorless, yellow, brown, gray, green, blue, pink, red, black. Colorless is most prized for gems.
• Luster: Adamantine (brilliant, like a diamond) to greasy (on rough or etched surfaces).
• Transparency: Transparent to opaque. Gem-quality diamonds are typically transparent.
• Fracture: Conchoidal to uneven. Despite its hardness, it can fracture.
• Cleavage: Perfect octahedral {111} in four directions.
• Composition: Pure carbon (C). Trace elements (e.g., nitrogen, boron) cause color variations.

Quick Check

• Color: Variable (colorless, yellow, brown most common)
• Luster: Adamantine to greasy
• Streak: Colorless (as it is pure carbon and harder than the streak plate)

Physical Characteristics

• Crystal Habit: Octahedral, dodecahedral, cubic, or combinations thereof. Also macles (twinned crystals), bort (irregular aggregates), and carbonado (polycrystalline aggregates).
• Cleavage Type: Perfect octahedral {111}. This means it breaks cleanly along four specific planes, forming smooth, flat surfaces.
• Fracture Type: Conchoidal (shell-like, curved breaks) to uneven. This occurs when the diamond breaks in directions other than its cleavage planes.
• Tenacity: Brittle. Despite its extreme hardness, diamond can be fractured or cleaved by a sharp blow.
• Luster Type: Adamantine (for well-formed, clean surfaces) to greasy (for rough, etched, or coated surfaces).

Formation

Diamonds form under conditions of extremely high pressure and temperature, typically at depths of 140 to 190 kilometers (87 to 118 miles) in the Earth's mantle. These conditions are found within the stable continental lithospheric mantle, often beneath ancient cratons. The carbon source is believed to be methane or carbon dioxide, or organic carbon from subducted oceanic crust. Diamonds are brought to the Earth's surface through deep-seated volcanic eruptions, primarily by kimberlite and lamproite magmas, which ascend rapidly, preventing the diamonds from reverting to graphite. Some diamonds can also form at impact sites (impact diamonds) or in ultra-high-pressure metamorphic rocks (e.g., ophiolites), though these are rare and typically microscopic.

Usage

Gem-quality diamonds are highly valued for jewelry due to their exceptional brilliance, fire, and hardness. Industrial-grade diamonds (bort, carbonado) are extensively used in cutting, grinding, drilling, and polishing tools due to their unparalleled hardness. They are critical in manufacturing abrasives, drill bits, saw blades, and wire drawing dies. Diamonds are also used in high-tech applications such as heat sinks in electronics, optical windows, and in quantum computing research.

Age Distribution

Diamonds range in age from approximately 990 million years to over 4.2 billion years, with most commercial diamonds being between 1 billion and 3.3 billion years old. They are among the oldest materials on Earth.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Native Elements

Group

Carbon Group

Crystal System

Cubic (Isometric)

Chemical Formula

C

Composition

Pure carbon. May contain trace amounts of nitrogen, boron, or other elements as impurities, which can influence color and other properties.

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