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Tiger's Eye is a metamorphic rock and a variety of chalcedony (cryptocrystalline quartz) known for its striking chatoyancy, or 'cat's eye' effect. This optical phenomenon is caused by the parallel intergrowth of quartz fibers and altered amphibole minerals (primarily crocidolite) that reflect light. Its typical colors range from golden-yellow to reddish-brown. Onyx is a cryptocrystalline variety of quartz, specifically a banded form of chalcedony. It is characterized by parallel bands of different colors, most commonly black and white, but can also include shades of brown, red, and green. Unlike agate, which has curved bands, onyx typically exhibits straight, parallel bands.
How to Identify
- Color
- 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.
- Luster
- Tiger's Eye: Silky to vitreous. Onyx: Waxy to vitreous.
- Texture
- Tiger's Eye: Fibrous, often with a smooth, polished feel. Onyx: Smooth, microcrystalline, often with a conchoidal fracture.
- Crystal Form
- 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.
- Cleavage
- Neither Tiger's Eye nor Onyx exhibit true cleavage due to their cryptocrystalline nature. They typically show conchoidal fracture.
- Geological Environment
- 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)
Quick Check
- Color: Tiger's Eye: Golden-brown, yellow, red-brown. Onyx: Black and white, or other banded colors.
- Luster: Tiger's Eye: Silky. Onyx: Waxy to vitreous.
- Streak: White (for both, as they are varieties of quartz).
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.
Formation
Tiger's Eye forms when crocidolite (a blue asbestos mineral) is replaced by quartz, while retaining the fibrous structure. Iron oxides (limonite) then stain the quartz, giving it its characteristic golden-brown color and chatoyancy. Onyx is a banded variety of chalcedony, typically forming in the gas cavities of lavas or as a deposit from silica-rich waters, often in stalactitic or stalagmitic forms. The banding in onyx is due to successive layers of silica deposition, often with varying impurities.
Usage
Both Tiger's Eye and Onyx are widely used as gemstones, for carving, ornamental objects, and in jewelry. Tiger's Eye is prized for its chatoyancy, while Onyx is valued for its contrasting bands and ability to be carved into cameos and intaglios.
Age Distribution
Tiger's Eye: Precambrian to Cenozoic; Onyx: Varies widely, often associated with volcanic activity or hot spring deposits.
Where to Find
Tiger's Eye: South Africa
The primary source of high-quality Tiger's Eye, particularly in the Griqualand West region.
Tiger's Eye: Australia
Significant deposits found in Western Australia.
Tiger's Eye: India
Known for some occurrences of Tiger's Eye.
Tiger's Eye: Myanmar (Burma)
Minor occurrences.
Tiger's Eye: United States
Limited deposits in various states.
Onyx: Brazil
A major source of various types of chalcedony, including onyx.
Onyx: Uruguay
Known for its agate and onyx deposits.
Onyx: Mexico
Significant deposits of banded onyx, often used for carvings.
Onyx: United States
Found in various states, particularly in the southwestern regions.
Onyx: India
Historical source of onyx.
Onyx: Pakistan
Known for its green onyx.
Finding Tips
Look for Chatoyancy (Tiger's Eye)
When searching for Tiger's Eye, look for the characteristic silky luster and the moving band of light (chatoyancy) that shifts as the stone is rotated. This is its most distinctive feature.
Examine Banding (Onyx)
For Onyx, focus on specimens with distinct, parallel bands of contrasting colors. The straightness and clarity of the bands are key identifying features.
Check Hardness
Both minerals are varieties of quartz, so they will scratch glass (Mohs hardness of 7). This can help differentiate them from softer look-alikes.
Consider Geological Context
Tiger's Eye is often found in metamorphic terrains associated with asbestos deposits. Onyx is typically found in volcanic regions, hot spring deposits, or sedimentary environments where silica-rich waters have precipitated.
Polishing Reveals Features
The chatoyancy of Tiger's Eye and the banding of Onyx are best observed on polished surfaces. Rough specimens may require careful examination.
Similar Rocks
Hawk's Eye
Fibrous chalcedony (quartz pseudomorph after crocidolite)
Also known as: Blue Tiger's Eye
Cat's Eye Chrysoberyl
Chrysoberyl with fibrous inclusions
Also known as: Cymophane
Agate
Cryptocrystalline quartz
Also known as: Banded Chalcedony
Sardonyx
Cryptocrystalline quartz (variety of onyx with sard and onyx bands)
Also known as: Banded Chalcedony
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group (Chalcedony Subgroup)
- Crystal System
- Trigonal (cryptocrystalline aggregates)
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide
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