How to identify Diamond and Sapphire

To correctly identify Diamond and Sapphire (Diamond (C) and Sapphire (Al2O3)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Diamond: Typically colorless to yellow, brown, or gray; rarely pink, blue, green, orange, red, or black. Sapphire: Most commonly blue; also pink, yellow, green, purple, orange, black, and colorless (fancy sapphires).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Diamond: Adamantine. Sapphire: Vitreous to sub-adamantine.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Both are crystalline with smooth, polished facets when cut. Natural crystals may show growth patterns or striations.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Diamond: Isometric system, typically octahedral, dodecahedral, or cubic crystals. Sapphire: Hexagonal (trigonal) system, typically barrel-shaped, bipyramidal, or tabular crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Diamond: Perfect octahedral cleavage in four directions. Sapphire: No true cleavage, but exhibits parting along basal planes and rhombohedral planes.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Diamond: Kimberlite and lamproite pipes (primary deposits), alluvial and marine placers (secondary deposits). Sapphire: Basaltic lavas, metamorphic rocks (marbles, schists, gneisses), pegmatites, and alluvial deposits.

Key Facts

Hardness

Diamond: 10 (Mohs scale, hardest natural mineral). Sapphire: 9 (Mohs scale).

Specific Gravity

Diamond: 3.50 - 3.53. Sapphire: 3.95 - 4.03.

Crystal System

Diamond: Isometric. Sapphire: Hexagonal (Trigonal).

Color

Diamond: Colorless, yellow, brown, gray, blue, green, pink, red, orange, black. Sapphire: Blue, pink, yellow, green, purple, orange, black, colorless.

Luster

Diamond: Adamantine. Sapphire: Vitreous to sub-adamantine.

Transparency

Diamond: Transparent to opaque. Sapphire: Transparent to opaque.

Fracture

Diamond: Conchoidal to uneven. Sapphire: Conchoidal to uneven.

Cleavage

Diamond: Perfect octahedral in four directions. Sapphire: None (exhibits parting).

Composition

Diamond: Pure Carbon (C). Sapphire: Aluminum Oxide (Al2O3) with trace elements causing color.

Physical characteristics

• Crystal Habit: Diamond: Octahedral, dodecahedral, cubic, or combinations; also massive, granular. Sapphire: Prismatic, tabular, bipyramidal, barrel-shaped; also massive, granular.
• Cleavage Type: Diamond: Perfect octahedral {111}. Sapphire: None, but parting on {0001} and {1011}.
• Fracture Type: Diamond: Conchoidal to uneven. Sapphire: Conchoidal to uneven.
• Tenacity: Diamond: Brittle. Sapphire: Brittle.
• Luster Type: Diamond: Adamantine. Sapphire: Vitreous to sub-adamantine.

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