How to identify Tiger's Eye and Onyx

To correctly identify Tiger's Eye and Onyx (Tiger's Eye (fibrous chalcedony, a variety of quartz) and Onyx (chalcedony, a cryptocrystalline variety of quartz)), 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

   Tiger's Eye: Golden-yellow, golden-brown, reddish-brown, sometimes blue (Hawk's Eye). Onyx: Typically black and white banded, but can also be brown, red, or green banded.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Tiger's Eye: Silky to vitreous. Onyx: Waxy to vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Tiger's Eye: Fibrous, often with a smooth, polished feel. Onyx: Smooth, microcrystalline, often with a conchoidal fracture.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Both are cryptocrystalline, meaning their crystals are too small to be seen without high magnification. Tiger's Eye exhibits a pseudomorphous fibrous structure. Onyx forms in banded layers.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Neither Tiger's Eye nor Onyx exhibit true cleavage due to their cryptocrystalline nature. They typically show conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Tiger's Eye: Formed in metamorphic rocks, often associated with crocidolite deposits. Onyx: Found in volcanic rocks (as amygdules or geodes), hot spring deposits, and as stalactitic/stalagmitic formations in caves.

Key Facts

Hardness

7 on the Mohs scale (for both)

Specific Gravity

2.64 - 2.71 (for both)

Crystal System

Trigonal (cryptocrystalline aggregates)

Color

Tiger's Eye: Golden-yellow, golden-brown, reddish-brown, blue. Onyx: Black and white, brown, red, green (banded).

Luster

Tiger's Eye: Silky to vitreous. Onyx: Waxy to vitreous.

Transparency

Opaque to translucent (for both)

Fracture

Conchoidal to uneven (for both)

Cleavage

None (for both)

Composition

Silicon dioxide (SiO2) with various impurities (e.g., iron oxides in Tiger's Eye)

Physical characteristics

• Crystal Habit: Tiger's Eye: Fibrous aggregates, pseudomorphous after crocidolite. Onyx: Microcrystalline aggregates, typically banded, often botryoidal or stalactitic.
• Cleavage Type: None
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Tiger's Eye: Silky to vitreous. Onyx: Waxy to vitreous.

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