How to identify Ruby in Zoisite

To correctly identify Ruby in Zoisite (Ruby (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

   Dominantly green (zoisite) with prominent opaque red (ruby) inclusions and often black (amphibole) patches.

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 and amphibole embedded in the zoisite matrix.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Ruby typically occurs as hexagonal prisms or irregular grains. Zoisite forms prismatic crystals, often massive. Amphibole forms acicular to prismatic crystals or irregular masses.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Zoisite: Perfect on {010}; Ruby: None (parting on {0001} and {1011}); Amphibole: Two distinct cleavages at approximately 56° and 124°.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Medium to high-grade regional metamorphic terrains, particularly in areas where chromium-rich protoliths (e.g., anorthosites, gabbros, or ultramafic rocks) have undergone metasomatism or metamorphism.

Key Facts

Hardness

Varies depending on the constituent minerals: Ruby (9), Zoisite (6-7), Amphibole (5-6). Overall rock hardness is intermediate.

Specific Gravity

Varies: Ruby (3.97-4.05), Zoisite (3.10-3.38), Amphibole (2.9-3.6). The rock's specific gravity will be an average of its components.

Crystal System

Ruby (Hexagonal), Zoisite (Orthorhombic), Amphibole (Monoclinic)

Color

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

Luster

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

Transparency

Ruby (Opaque in this form), Zoisite (Translucent to opaque), Amphibole (Opaque)

Fracture

Ruby (Conchoidal to uneven), Zoisite (Uneven to conchoidal), Amphibole (Uneven to subconchoidal)

Cleavage

Ruby (None, but parting), Zoisite (Perfect on {010}), Amphibole (Two distinct cleavages at ~56° and ~124°)

Composition

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

Physical characteristics

• Crystal Habit: Ruby: Hexagonal prisms, granular; Zoisite: Prismatic, massive; Amphibole: Acicular, prismatic, massive.
• Cleavage Type: Zoisite: Perfect basal; Ruby: Parting, not true cleavage; Amphibole: Prismatic.
• Fracture Type: Conchoidal to uneven for ruby and zoisite, uneven to subconchoidal for amphibole.
• Tenacity: Brittle for all constituent minerals.
• Luster Type: Vitreous to pearly for zoisite, adamantine to vitreous for ruby, vitreous to dull for amphibole.

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