How to identify Hawk's Eye

To correctly identify Hawk's Eye (Silicified Crocidolite (Quartz variety)), 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

   Dark blue to bluish-gray, sometimes with greenish or brownish streaks. The blue color is distinct from the golden-brown of Tiger's Eye.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Silky to vitreous, exhibiting a strong chatoyancy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fine-grained, fibrous appearance internally, though the surface is smooth when polished.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

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

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is a cryptocrystalline quartz. The original crocidolite would have perfect prismatic cleavage, but this is replaced.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in metamorphosed banded iron formations (BIFs) where crocidolite-rich layers have undergone silicification. These environments are often associated with ancient sedimentary basins.

Key Facts

Hardness

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

Specific Gravity

2.64-2.71

Crystal System

Trigonal (for quartz), but pseudomorphs original monoclinic crocidolite structure

Color

Dark blue, bluish-gray, sometimes with greenish or brownish tints

Luster

Silky to vitreous, chatoyant

Transparency

Opaque

Fracture

Conchoidal to splintery

Cleavage

None (for quartz)

Composition

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

Physical characteristics

• Crystal Habit: Massive, fibrous aggregates, pseudomorphing original crocidolite
• Cleavage Type: None (due to quartz replacement)
• Fracture Type: Conchoidal to splintery
• Tenacity: Brittle
• Luster Type: Silky to vitreous

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