How to identify Black Spinel

To correctly identify Black Spinel (Spinel), 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

   Typically opaque black, sometimes with a very dark brown or purplish tint when viewed under strong light.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to sub-adamantine.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly conchoidal fracture surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Crystals are typically octahedral, sometimes dodecahedral, or a combination. Often found as well-formed euhedral crystals. Can also occur as granular masses.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Poor to indistinct octahedral cleavage, often appearing as parting rather than true cleavage. Fracture is more prominent.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks (e.g., marbles, gneisses, schists), mafic and ultramafic igneous rocks (e.g., peridotites, basalts), and associated placer deposits.

Key Facts

Hardness

8 on Mohs scale

Specific Gravity

3.58 - 4.10 (varies with iron content)

Crystal System

Isometric (Cubic)

Color

Opaque black, sometimes with very dark brown or purplish undertones

Luster

Vitreous to sub-adamantine

Transparency

Opaque

Fracture

Conchoidal to uneven

Cleavage

Poor to indistinct octahedral, often parting

Composition

Magnesium Aluminum Oxide (MgAl2O4) with significant iron (Fe) substitution, often forming a solid solution towards Hercynite (FeAl2O4)

Physical characteristics

• Crystal Habit: Octahedral, dodecahedral, or granular masses. Often twinned (spinel law).
• Cleavage Type: Poor to indistinct octahedral {111} parting, not true cleavage.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy) to sub-adamantine.

Explore Black Spinel