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Diamond is a metastable allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond cubic. It is the hardest known natural material, with a Mohs hardness of 10. Its exceptional hardness, combined with its high refractive index and dispersion, gives it its characteristic brilliance and fire when cut as a gemstone. Diamonds are typically colorless, but impurities can lead to a range of colors, including yellow, brown, green, blue, pink, red, and black. They are chemically inert and excellent electrical insulators, though some blue diamonds containing boron can be semiconductors.
How to Identify
- Color
- Typically colorless, but can be yellow, brown, green, blue, pink, red, or black due to impurities or structural defects.
- Luster
- Adamantine (brilliant, like a diamond).
- Texture
- Smooth to slightly rough on natural crystal faces; fractured surfaces are conchoidal.
- Crystal Form
- Cubic system, commonly found as octahedra, dodecahedra, cubes, or combinations thereof. Macles (twinned crystals) are also common.
- Cleavage
- Perfect in four directions (octahedral), meaning it can be split along these planes, despite its extreme hardness.
- Geological Environment
- Primarily found in kimberlite and lamproite pipes, which are volcanic conduits that bring diamonds from the mantle to the surface. Also found in alluvial (placer) deposits, where diamonds have been eroded from their primary sources and transported by rivers.
Key Facts
- Hardness: 10 (Mohs scale), the hardest natural mineral
- Specific Gravity: 3.50 - 3.53
- Crystal System: Cubic (isometric)
- Color: Colorless, yellow, brown, green, blue, pink, red, black
- Luster: Adamantine
- Transparency: Transparent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: Perfect in four directions (octahedral)
- Composition: Pure carbon (C)
Quick Check
- Color: Colorless to various colors
- 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) and bort (poorly crystallized or industrial grade).
- Cleavage Type: Octahedral, perfect on {111} planes.
- Fracture Type: Conchoidal to uneven, brittle.
- Tenacity: Brittle, despite extreme hardness.
- Luster Type: Adamantine (diamond-like).
Formation
Diamonds form under extreme pressure and high temperature conditions deep within the Earth's mantle, typically at depths of 140 to 190 kilometers (87 to 118 miles). These conditions are found in the cratonic roots of continental plates. The carbon atoms are bonded in a strong, rigid tetrahedral lattice. They are brought to the Earth's surface rapidly through explosive volcanic eruptions of kimberlite and lamproite magmas, which originate in the mantle.
Usage
Diamonds are highly valued as gemstones for jewelry due to their exceptional brilliance, fire, and hardness. Industrially, their extreme hardness makes them indispensable for cutting, grinding, drilling, and polishing tools. They are used in abrasive powders, drill bits, saw blades, and in high-precision machining. Synthetic diamonds are also produced for these industrial applications and increasingly for technological uses in electronics and optics.
Age Distribution
Diamonds typically range in age from 990 million to 4.2 billion years old, predating the kimberlite or lamproite eruptions that bring them to the surface.
Where to Find
Botswana
Leading producer of gem-quality diamonds, primarily from kimberlite pipes like Orapa, Jwaneng, and Letlhakane.
Russia
Significant producer of both gem and industrial diamonds, with major kimberlite pipes in Siberia (e.g., Mir, Udachnaya).
Canada
Relatively new but significant producer, with major kimberlite mines in the Northwest Territories (e.g., Ekati, Diavik, Gahcho Kué).
Australia
Historically a major producer, particularly of fancy colored diamonds (pink, red) from the Argyle lamproite mine (now closed), and industrial diamonds.
South Africa
Historically the most famous diamond-producing country, with iconic kimberlite pipes like Kimberley (Big Hole) and Cullinan (Premier).
Angola
Significant producer of both alluvial and kimberlite diamonds.
Finding Tips
Primary Deposits (Kimberlite/Lamproite Pipes)
Diamonds are found within these igneous rocks. Prospecting involves identifying these specific rock types, often through geophysical surveys and indicator mineral sampling (e.g., pyrope garnet, ilmenite, chromite).
Secondary Deposits (Alluvial/Placer Deposits)
Diamonds, being dense and resistant to weathering, accumulate in riverbeds, ancient river channels, and coastal areas. Prospecting involves sampling gravels and concentrates for diamonds. Look for areas downstream from known or suspected primary sources.
Indicator Minerals
When prospecting for diamonds, look for 'indicator minerals' such as pyrope garnet (often purple-red), chromite, ilmenite, and chrome diopside. These minerals are commonly found alongside diamonds in kimberlite and lamproite and are more abundant and easier to spot.
Density Separation
Diamonds have a relatively high specific gravity (3.5-3.53). In alluvial settings, panning or sluicing can help concentrate heavier minerals, including diamonds, from lighter sediments.
UV Fluorescence
Some diamonds fluoresce under ultraviolet light (typically blue, but can be other colors). This can be a useful, though not definitive, identification tool.
Similar Rocks
Moissanite
Moissanite (SiC)
Also known as: Silicon Carbide
Cubic Zirconia
Zirconium Dioxide (ZrO2)
Also known as: CZ
White Sapphire
Corundum (Al2O3)
Also known as: Colorless Corundum
Scientific Classification
- Mineral Class
- Native Elements
- Group
- Carbon Group
- Crystal System
- Cubic (Isometric)
- Chemical Formula
- C
- Composition
- Pure carbon
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