Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Tiger Iron is a visually striking composite rock characterized by its alternating bands of golden-brown chatoyant tiger's eye, metallic gray to black hematite, and rich red to brown jasper. It is a variety of banded iron formation (BIF) that has undergone specific geological processes to incorporate the fibrous quartz of tiger's eye. The interplay of these three distinct minerals creates a unique aesthetic with contrasting colors, lusters, and textures. The tiger's eye component exhibits a characteristic silky luster and chatoyancy (a mobile, shimmering optical effect), while hematite provides a submetallic to metallic sheen, and jasper contributes a dull to vitreous luster.
How to Identify
- Color
- Distinctive alternating bands of golden-brown (tiger's eye), metallic gray to black (hematite), and red to brown (jasper).
- Luster
- Varies within the rock: silky to chatoyant (tiger's eye), submetallic to metallic (hematite), dull to vitreous (jasper).
- Texture
- Banded, often with a fine-grained to microcrystalline texture. The tiger's eye component may show a fibrous texture.
- Crystal Form
- Not applicable as a composite rock; individual mineral components are microcrystalline (jasper, tiger's eye) or massive/granular (hematite).
- Cleavage
- None observed in the composite rock. Individual minerals have different cleavage properties (e.g., hematite has no true cleavage but parting).
- Geological Environment
- Formed within ancient banded iron formations (BIFs) that have undergone subsequent metasomatic alteration and low-grade metamorphism. Found in regions with extensive Precambrian shield rocks.
Key Facts
- Hardness: 6.5-7 on Mohs scale (due to quartz/jasper components), but can be slightly lower in hematite-rich areas (5-6)
- Specific Gravity: Approximately 3.3-4.0 (variable due to different mineral densities; hematite is denser than quartz)
- Crystal System: Not applicable as a composite rock. Quartz is trigonal, Hematite is trigonal.
- Color: Banded golden-brown, metallic gray/black, red/brown
- Luster: Mixed: silky/chatoyant, submetallic/metallic, dull/vitreous
- Transparency: Opaque
- Fracture: Conchoidal to uneven (due to quartz/jasper), splintery (due to fibrous tiger's eye)
- Cleavage: None (in the composite rock)
- Composition: A composite of microcrystalline quartz (jasper), fibrous quartz pseudomorph after crocidolite (tiger's eye), and iron oxide (hematite).
Quick Check
- Color: Banded golden-brown, metallic gray/black, red/brown
- Luster: Mixed: silky/chatoyant, submetallic/metallic, dull/vitreous
- Streak: Reddish-brown (from hematite component)
Physical Characteristics
- Crystal Habit: Massive, banded, microcrystalline, fibrous (tiger's eye component)
- Cleavage Type: None
- Fracture Type: Conchoidal to uneven, splintery
- Tenacity: Brittle
- Luster Type: Mixed: silky, chatoyant, submetallic, metallic, dull, vitreous
Formation
Tiger Iron forms from banded iron formations (BIFs), which are distinctive sedimentary rocks consisting of alternating layers of iron-rich minerals (like hematite and magnetite) and chert (microcrystalline quartz, often colored as jasper). The tiger's eye component (fibrous crocidolite replaced by quartz) is typically introduced through later metasomatic alteration and silicification of specific iron-rich layers within these BIFs, often under low-grade metamorphic conditions. The characteristic banding is a primary sedimentary feature, while the chatoyancy of the tiger's eye is a secondary metamorphic/metasomatic feature.
Usage
Primarily used as an ornamental stone, for lapidary purposes (cabochons, beads, carvings), and in jewelry. Its striking appearance makes it popular for decorative items and as a collector's mineral. It is also sometimes used in metaphysical practices.
Age Distribution
Predominantly Precambrian (specifically Paleoproterozoic to Neoarchean), ranging from approximately 1.8 to 3.8 billion years old.
Where to Find
Pilbara Craton, Western Australia
The primary and most significant source of high-quality Tiger Iron. The banded iron formations here are among the oldest on Earth, dating back to the Archean Eon.
South Africa
Another notable source, particularly in the Northern Cape Province, where extensive banded iron formations occur.
United States (Michigan, Minnesota)
While not typically producing the same quality or quantity of ornamental Tiger Iron, banded iron formations exist in the Lake Superior region, which can contain similar mineral assemblages, though often lacking the distinct tiger's eye component.
Finding Tips
Geological Context
Look for areas known for ancient banded iron formations (BIFs), particularly those that have experienced low-grade metamorphism and metasomatic alteration.
Visual Identification
Search for rocks exhibiting distinct, parallel bands of golden-brown, metallic gray/black, and red/brown. The chatoyancy of the tiger's eye component is a key indicator.
Associated Minerals
Tiger Iron will be found in association with other iron-rich minerals, chert, and sometimes other metamorphic minerals depending on the specific BIF and metamorphic grade.
Safety Precautions
When collecting in the field, be aware of potential hazards associated with old mining sites or rugged terrain. Always wear appropriate safety gear, including eye protection and sturdy footwear. While Tiger Iron itself is not acutely hazardous, some associated minerals in BIFs (like crocidolite asbestos, the precursor to tiger's eye) can be dangerous if disturbed or inhaled. However, in Tiger Iron, the crocidolite has been fully replaced by quartz, rendering it safe.
Similar Rocks
Tiger's Eye
Quartz pseudomorph after Crocidolite
Also known as: Crocodilite Asbestos (original form)
Jasper
Microcrystalline Quartz
Also known as: Chert, Flint
Hematite
Iron Oxide (Fe2O3)
Also known as: Kidney Ore, Specularite
Banded Iron Formation (BIF)
Chemically precipitated sedimentary rock
Also known as: Taconite
Scientific Classification
- Mineral Class
- Oxides (Hematite), Silicates (Quartz/Jasper/Tiger's Eye)
- Group
- Composite Rock (Banded Iron Formation variety)
- Crystal System
- Not applicable for the composite. Quartz: Trigonal; Hematite: Trigonal.
- Chemical Formula
- Variable, primarily SiO2 (quartz/jasper/tiger's eye) and Fe2O3 (hematite), with minor impurities.
- Composition
- A natural aggregate of quartz (SiO2), hematite (Fe2O3), and sometimes goethite (FeO(OH)). The quartz exists as microcrystalline jasper and as fibrous tiger's eye (silicified crocidolite).
Explore Tiger Iron
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.