How to identify Ruby in Zoisite

To correctly identify Ruby in Zoisite (Corundum in Zoisite), 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

   Distinctive combination of opaque green (zoisite), opaque red (ruby), and black (amphibole).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Zoisite: Vitreous to pearly; Ruby: Adamantine to vitreous; Amphibole: Vitreous to dull.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Granular to massive, often with visible ruby crystals embedded within the zoisite matrix.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Ruby typically occurs as hexagonal prisms or irregular grains. Zoisite is massive or columnar. Amphibole is acicular or massive.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Zoisite: Perfect on {010}; Ruby: No true cleavage, but parting on {0001} and {1011}; Amphibole: Two distinct cleavages at approximately 56 and 124 degrees.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metasomatized mafic or ultramafic rocks, often associated with regional metamorphic terrains, particularly in areas with chromium enrichment.

Key Facts

Hardness

Variable, 6-7 for zoisite, 9 for ruby, 5-6 for amphibole (Mohs scale)

Specific Gravity

Variable, 3.1-3.4 for zoisite, 3.9-4.1 for ruby, 3.0-3.4 for amphibole

Crystal System

Zoisite: Orthorhombic; Ruby (Corundum): Trigonal; Amphibole: Monoclinic

Color

Green (zoisite), Red (ruby), Black (amphibole)

Luster

Vitreous to pearly (zoisite), Adamantine to vitreous (ruby), Vitreous to dull (amphibole)

Transparency

Opaque to translucent (zoisite), Opaque (ruby), Opaque (amphibole)

Fracture

Uneven to conchoidal (zoisite), Conchoidal to uneven (ruby), Uneven (amphibole)

Cleavage

Zoisite: Perfect on {010}; Ruby: No true cleavage, parting on {0001} and {1011}; Amphibole: Two distinct cleavages at approximately 56 and 124 degrees

Composition

A rock composed of Zoisite (Ca2Al3(SiO4)3(OH)), Corundum (Al2O3, specifically ruby due to Cr3+ impurities), and Amphibole (complex silicates, e.g., pargasite (NaCa2(Mg4Al)Si6Al2O22(OH)2) or hornblende).

Physical characteristics

• Crystal Habit: Zoisite: Prismatic, columnar, massive; Ruby: Hexagonal prisms, tabular, massive; Amphibole: Prismatic, acicular, massive.
• Cleavage Type: Zoisite: Perfect; Ruby: Parting; Amphibole: Distinct.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly (zoisite), Adamantine to vitreous (ruby), Vitreous to dull (amphibole).

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