How to identify Ruby in Zoisite

To correctly identify Ruby in Zoisite (Ruby (Corundum) and 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), black (amphibole), and pink to deep red (ruby). The green zoisite can vary in intensity, and the ruby can appear as scattered crystals or larger masses.

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 individual crystals of ruby, zoisite, and amphibole. The texture can be uneven due to the varying hardness and crystal habits of the constituent minerals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Ruby: Hexagonal prisms, often anhedral to subhedral within the matrix. Zoisite: Prismatic, often massive. Amphibole: Prismatic to acicular.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic terrains, particularly in areas of regional metamorphism and metasomatism involving mafic and ultramafic rocks. Often found in association with chromium-rich rocks.

Key Facts

Hardness

Ruby: 9 (Mohs). Zoisite: 6-6.5 (Mohs). Amphibole: 5-6 (Mohs).

Specific Gravity

Ruby: 3.95-4.10. Zoisite: 3.10-3.38. Amphibole: 2.9-3.6.

Crystal System

Ruby: Trigonal. Zoisite: Orthorhombic. Amphibole: Monoclinic.

Color

Green (zoisite), black (amphibole), red/pink (ruby).

Luster

Vitreous to pearly (zoisite), adamantine to vitreous (ruby), vitreous to dull (amphibole).

Transparency

Opaque to translucent.

Fracture

Ruby: Uneven to conchoidal. Zoisite: Uneven. Amphibole: Uneven to subconchoidal.

Cleavage

Ruby: No true cleavage, but basal parting. Zoisite: Perfect on {010}. Amphibole: Two distinct cleavages at ~56° and ~124°.

Composition

A rock composed of Ruby (Al2O3), Zoisite (Ca2Al3(SiO4)3(OH)), and various amphiboles (e.g., Pargasite, Hornblende - complex silicates).

Physical characteristics

• Crystal Habit: Ruby: Hexagonal prisms, often anhedral to subhedral. Zoisite: Prismatic, massive. Amphibole: Prismatic, acicular.
• Cleavage Type: Ruby: Parting. Zoisite: Perfect. 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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