Diamond

Description

Diamond is a metastable allotrope of carbon, where carbon atoms are arranged in a crystal structure called a diamond cubic. This structure is responsible for diamond's extraordinary physical properties, including its extreme hardness, high thermal conductivity, and high refractive index. It is the hardest known natural material. Diamonds are typically transparent and colorless, but impurities or structural defects can result in a wide range of colors, including yellow, brown, green, blue, pink, red, and black. Its exceptional optical properties, including high dispersion (fire) and adamantine luster, make it a highly prized gemstone. Diamond is chemically inert to most acids and bases at room temperature.

How to Identify

Color

Colorless, yellow, brown, green, blue, pink, red, orange, black. The most common colors are colorless to light yellow/brown. Fancy colors are rare and highly valued.

Luster

Adamantine (brilliant, like a polished metal, but with a glassy appearance).

Texture

Smooth to slightly rough on unpolished surfaces, often exhibiting growth features like trigons (triangular etch pits) on octahedral faces.

Crystal Form

Typically forms isometric crystals, most commonly octahedra, dodecahedra, and cubes, or combinations thereof. Macles (twinned crystals) are also common. Industrial diamonds can be anhedral (irregularly shaped).

Cleavage

Perfect octahedral cleavage in four directions {111}. This means it can be split along these planes, despite its extreme hardness.

Geological Environment

Found in kimberlite and lamproite pipes (primary deposits), which are volcanic conduits that brought diamonds from the mantle to the surface. Also found in alluvial (placer) deposits (secondary deposits) where diamonds have been eroded from their primary source and transported by rivers and streams.

Key Facts

• Hardness: 10 (Mohs scale), the hardest known natural mineral.
• Specific Gravity: 3.50 - 3.53 g/cm³ (varies slightly with impurities).
• Crystal System: Isometric (Cubic)
• Color: Colorless, yellow, brown, green, blue, pink, red, orange, black. Most commonly colorless to light yellow/brown.
• Luster: Adamantine
• Transparency: Transparent to opaque (depending on quality and inclusions).
• Fracture: Conchoidal to uneven.
• Cleavage: Perfect octahedral {111} in four directions.
• Composition: Pure Carbon (C)

Quick Check

• Color: Colorless to various hues (yellow, brown, blue, pink, etc.)
• Luster: Adamantine
• Streak: Colorless (as it is harder than the streak plate)

Physical Characteristics

• Crystal Habit: Octahedral, dodecahedral, cubic, or combinations; also macles (twinned crystals), bort (poorly crystallized or industrial grade), and carbonado (polycrystalline aggregates).
• Cleavage Type: Perfect, octahedral {111}.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle (despite extreme hardness, it can be fractured or cleaved by impact along cleavage planes).
• Luster Type: Adamantine.

Formation

Diamonds form under extreme conditions of high pressure and high temperature deep within the Earth's mantle, typically at depths of 150 to 200 kilometers (90 to 120 miles). The required conditions are pressures of 4.5 to 6 GPa (gigapascals) and temperatures of 900 to 1300 °C (1650 to 2370 °F). These conditions are found within the stable lithospheric mantle beneath ancient continental cratons. 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. These magmas form pipe-like intrusions called kimberlite pipes or lamproite pipes, which are the primary source rocks for diamonds.

Usage

Diamonds are highly valued for both industrial and gemological purposes. Gem-quality diamonds are used in jewelry due to their exceptional brilliance, fire, and hardness. Industrial-grade diamonds, which may be smaller, imperfect, or off-color, are indispensable in cutting, grinding, drilling, and polishing tools due to their unparalleled hardness. They are used in drill bits for oil and gas exploration, saw blades for concrete and stone, grinding wheels, and polishing compounds. Diamond anvils are used in high-pressure research. Synthetic diamonds are also produced for both industrial and, increasingly, gemological applications.

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. This indicates formation during various periods of Earth's history, primarily in the Archean and Proterozoic eons.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Native Elements

Group

Carbon Group

Crystal System

Isometric (Cubic)

Chemical Formula

C

Composition

Pure Carbon

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