metamorphic
Tiger iron slab (likely polished banded iron formation)
Banded iron formation (BIF), chiefly hematite/magnetite, quartz, and iron-rich amphibole or jasper
AI-generated identification · may be incorrectDark brown-black, subtly metallic to earthy luster with intricate red-brown and gray banding; hardness about 5.5–7, depending on quartz and iron minerals. The apparent curved bands are metamorphically folded layers, not crystal cleavage.
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Physical properties
Dark brown-black, subtly metallic to earthy luster with intricate red-brown and gray banding; hardness about 5.5–7, depending on quartz and iron minerals. The apparent curved bands are metamorphically folded layers, not crystal cleavage.
Formation & geological history
Formed from chemical sedimentary iron- and silica-rich deposits, commonly about 2.5–1.8 billion years old, then folded and metamorphosed. The specimen appears cut or polished, exposing the banded interior.
Uses & applications
Used for ornamental slabs, carvings, cabochons, and collector specimens; dense iron-rich varieties are less commonly used as ore than conventional BIF.
Geological facts
The pattern resembles tiger iron, a decorative variety of BIF often containing hematite, jasper, and chatoyant crocidolite or amphibole. Strong magnetism, if present, indicates abundant magnetite.
Field identification & locations
Identify by high density, alternating iron-rich and silica-rich bands, possible magnetism, and lack of vesicles; a streak test and magnet are useful. Classic tiger iron is associated with Western Australia, while BIF occurs worldwide.
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