metamorphic
Polished banded ironstone (likely magnetite-rich)
Banded iron formation (BIF), chiefly quartz/chert and magnetite (Fe₃O₄), possibly hematite (Fe₂O₃)
AI-generated identification · may be incorrectDark metallic-gray and tan bands with a smooth, polished surface; opaque, fine-grained, and commonly weakly magnetic. Quartz-rich portions are about Mohs 7, while magnetite is about 5.5–6; cleavage is absent and specific gravity is commonly 3.0–4.5.
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Physical properties
Dark metallic-gray and tan bands with a smooth, polished surface; opaque, fine-grained, and commonly weakly magnetic. Quartz-rich portions are about Mohs 7, while magnetite is about 5.5–6; cleavage is absent and specific gravity is commonly 3.0–4.5.
Formation & geological history
Formed from chemical sedimentary iron- and silica-rich layers deposited in Precambrian seas, then compacted, metamorphosed, and later polished by abrasion. Most BIF formed roughly 2.5–1.8 billion years ago during oxygenation of Earth’s oceans.
Uses & applications
Used as an ornamental and lapidary stone for cabochons, bookends, carvings, and polished specimens; iron-rich BIF is also an important industrial iron ore when economically concentrated.
Geological facts
The alternating bands record repeated precipitation of silica and iron minerals. A strong magnet response supports magnetite-rich BIF; a red-brown streak or coating indicates hematite.
Field identification & locations
Common sources include Michigan and Minnesota, the Lake Superior region, Australia, Brazil, India, and South Africa. Test magnetism and streak, and inspect broken edges for hard, glassy quartz/chert bands.
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