How to identify Blue Tiger's Eye

To correctly identify Blue Tiger's Eye (Crocidolite in Quartz (variety of Tiger's Eye)), 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

   Ranges from dark blue to bluish-gray, sometimes with hints of green or brown. The blue color is often streaky or banded.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Silky to vitreous, exhibiting a strong chatoyancy when polished, appearing as a moving band of light.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fine-grained, fibrous structure visible under magnification, often appearing as parallel fibers.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Massive, fibrous aggregates, pseudomorphing the original crocidolite fibers. Does not form distinct euhedral crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None (as quartz), but the fibrous structure of the original crocidolite can create a pseudo-cleavage or parting along the fiber direction.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in highly metamorphosed iron-rich sedimentary rocks, particularly banded iron formations, where crocidolite has formed and subsequently been replaced by quartz.

Key Facts

Hardness

6.5-7 on the Mohs scale (due to quartz).

Specific Gravity

2.64-2.71 g/cm³.

Crystal System

Trigonal (for quartz), but the original crocidolite was monoclinic.

Color

Blue, bluish-gray, sometimes with greenish or brownish tints.

Luster

Silky to vitreous, chatoyant.

Transparency

Opaque to translucent.

Fracture

Conchoidal to splintery (due to fibrous structure).

Cleavage

None (quartz), but pseudo-cleavage along fiber direction.

Composition

SiO2 (quartz) with inclusions of fibrous crocidolite (Na2(Fe2+,Mg)3Fe3+2Si8O22(OH)2).

Physical characteristics

• Crystal Habit: Massive, fibrous aggregates, pseudomorphing original crocidolite.
• Cleavage Type: None (quartz), but parting along fiber direction.
• Fracture Type: Conchoidal to splintery.
• Tenacity: Brittle.
• Luster Type: Silky to vitreous.

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