How to identify Ruby in matrix

To correctly identify Ruby in matrix (Corundum (Al2O3) in host rock), 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

   Ruby crystals are typically red, ranging from pinkish-red to deep purplish-red. The matrix color varies widely depending on the host rock (e.g., white for marble, dark for amphibolite, grey/green for schist).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Ruby crystals exhibit a vitreous to adamantine luster. The matrix luster will vary (e.g., dull to pearly for schist, vitreous for marble).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The texture of the matrix can be granular (marble), foliated (schist, gneiss), or massive (basalt). Ruby crystals themselves are typically hard and smooth.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Ruby crystals often form as hexagonal prisms, tabular crystals, or barrel-shaped crystals. They can be euhedral (well-formed) or anhedral (irregularly shaped) within the matrix.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Ruby (corundum) has no true cleavage but exhibits a parting parallel to the basal plane and rhombohedral faces due to lamellar twinning. The matrix minerals will have their own characteristic cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Ruby in matrix is found in various geological environments, including metamorphic terrains (marbles, gneisses, schists formed from aluminum-rich sediments or igneous rocks), and less commonly in igneous rocks (alkaline basalts, lamprophyres, or pegmatites). The presence of chromium is crucial for the red color.

Key Facts

Hardness

9 on the Mohs scale (for ruby); matrix minerals will vary.

Specific Gravity

3.95-4.10 (for ruby); matrix minerals will vary.

Crystal System

Trigonal (for ruby); matrix minerals will vary.

Color

Red (ruby); matrix color is highly variable.

Luster

Vitreous to adamantine (ruby); variable (matrix).

Transparency

Transparent to opaque (ruby); variable (matrix).

Fracture

Conchoidal to uneven (ruby); variable (matrix).

Cleavage

No true cleavage, but exhibits parting (ruby); variable (matrix).

Composition

Aluminum oxide (Al2O3) with trace chromium (Cr) for ruby; matrix composition is highly variable depending on the host rock.

Physical characteristics

• Crystal Habit: Hexagonal prisms, tabular, barrel-shaped (ruby); granular, foliated, massive (matrix).
• Cleavage Type: Parting on {0001} and {1011} (ruby); variable (matrix).
• Fracture Type: Conchoidal to uneven (ruby); variable (matrix).
• Tenacity: Brittle (ruby); variable (matrix).
• Luster Type: Vitreous to adamantine (ruby); variable (matrix).

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