Black Diamond

Description

Black diamond refers to two distinct types of diamonds: 1. Carbonado: A natural polycrystalline diamond, typically opaque, with a porous structure and a color ranging from dark grey to black. It is composed of a random aggregate of microscopic diamond crystals, graphite, and amorphous carbon. Carbonados are exceptionally tough due to their polycrystalline nature, making them highly resistant to breakage. 2. Opaque black diamonds: These are monocrystalline diamonds that owe their black color to numerous dark inclusions (e.g., graphite, hematite, iron oxides, sulfides) and/or structural defects within the diamond lattice. These inclusions are so dense that they render the diamond opaque and black. Both types are distinct from treated black diamonds, which are typically lower-quality colorless diamonds that have been irradiated and/or heat-treated to achieve a black color.

How to Identify

Color

Opaque black to dark grey. The color is uniform throughout for carbonados, while inclusion-rich black diamonds may show variations or a slightly mottled appearance under magnification.

Luster

Sub-metallic to adamantine, often duller than typical gem diamonds due to porosity (carbonado) or dense inclusions.

Texture

Carbonados are typically rough, porous, and irregular in shape, often resembling charcoal or clinker. Inclusion-rich black diamonds can have a smoother surface if polished, but their internal texture is dense with inclusions.

Crystal Form

Carbonados are polycrystalline aggregates, lacking a distinct external crystal form. Inclusion-rich black diamonds can occur as octahedral, dodecahedral, or cubic crystals, but their external form is often obscured by their opaque nature and rough surfaces.

Cleavage

None for carbonado due to its polycrystalline nature. For inclusion-rich black diamonds, perfect octahedral cleavage {111} is present but rarely observed due to their toughness and dense inclusions.

Geological Environment

Carbonados are found in alluvial deposits, primarily in Brazil and the Central African Republic, suggesting a secondary depositional environment after their primary formation. Inclusion-rich black diamonds are found in kimberlite and lamproite pipes, similar to other gem diamonds, indicating a mantle origin.

Key Facts

• Hardness: 10 (Mohs scale)
• Specific Gravity: 3.10 - 3.53 g/cm³ (lower for porous carbonados, higher for dense monocrystalline diamonds)
• Crystal System: Isometric (cubic)
• Color: Opaque black to dark grey
• Luster: Sub-metallic to adamantine, often dull
• Transparency: Opaque
• Fracture: Conchoidal to uneven (for monocrystalline diamond); irregular, granular (for carbonado)
• Cleavage: Perfect octahedral {111} (for monocrystalline diamond); none (for carbonado)
• Composition: Pure carbon (C)

Quick Check

• Color: Opaque black to dark grey
• Luster: Sub-metallic to adamantine, often dull
• Streak: Colorless (will scratch a streak plate, but leaves no colored powder)

Physical Characteristics

• Crystal Habit: Carbonado: Polycrystalline aggregate, irregular masses. Inclusion-rich black diamond: Octahedral, dodecahedral, or cubic, often distorted or rounded.
• Cleavage Type: Perfect octahedral {111} (for monocrystalline diamond); absent (for carbonado).
• Fracture Type: Conchoidal to uneven (for monocrystalline diamond); irregular, granular (for carbonado).
• Tenacity: Brittle (for monocrystalline diamond); exceptionally tough (for carbonado due to its polycrystalline nature).
• Luster Type: Sub-metallic to adamantine, often dull.

Formation

Black diamonds, particularly carbonados, have a debated origin. Unlike typical gem-quality diamonds that form deep within the Earth's mantle under extreme pressure and temperature (150-200 km depth, 900-1300 °C) and are brought to the surface by kimberlite or lamproite volcanic eruptions, carbonados are thought to have a different genesis. Leading hypotheses for carbonado formation include: 1. Impactogenesis: Formation during meteorite impacts on Earth's surface. 2. Deep mantle origin: Formation at extreme depths, possibly deeper than typical diamonds, with subsequent transport. 3. Extraterrestrial origin: Formation in space, possibly within supernovae, and delivered to Earth via meteorites. This hypothesis is supported by their unique isotopic signatures (e.g., high 13C/12C ratios) and the presence of trace elements not typically found in terrestrial diamonds. Their porous, polycrystalline structure also differentiates them from monocrystalline gem diamonds. Other black diamonds, which are not carbonados, derive their color from abundant dark inclusions (e.g., graphite, sulfides, other opaque minerals) within a typical diamond crystal lattice, forming under standard mantle conditions.

Usage

Historically, black diamonds (carbonados) were primarily used for industrial purposes due to their extreme hardness and polycrystalline nature, making them excellent abrasives for drilling, cutting, and grinding tools. In recent decades, their unique aesthetic has led to increased use in high-end jewelry, particularly for engagement rings and designer pieces. They are also used in scientific research due to their unique properties.

Age Distribution

Precambrian to Phanerozoic, with some carbonados estimated to be 2.6 to 3.8 billion years old.

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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