sedimentary

Polished banded ironstone pebble

Banded iron formation (BIF), chiefly hematite/magnetite and quartz

AI-generated identification · may be incorrect

Dark metallic gray to black with rusty-brown iron-oxide bands; polished surface, opaque, fine-grained, and typically magnetic if magnetite-rich. Hardness is about 5.5–6.5 (hematite/magnetite) and 7 for quartz; specific gravity commonly 3.0–5.2.

Identified September 13, 2026More sedimentary →

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Physical properties

Formation & geological history

Chemical sediment formed when ancient oceans precipitated dissolved iron and silica, commonly around 2.5–1.8 billion years ago during Earth’s oxygenation. Later compaction, lithification, and weathering produced the banding and reddish oxidation.

Uses & applications

Used as iron ore when sufficiently iron-rich; attractive polished pieces are sold as lapidary stones, cabochons, and curiosities.

Geological facts

The alternating layers record repeated changes in ocean chemistry and oxygen availability. Hematite-rich varieties may show a red-brown streak, while magnetite-rich material attracts a magnet.

Field identification & locations

Identify by thin contrasting iron-rich and silica-rich bands, heavy weight, metallic sheen, and possible magnetism; common sources include Australia, Brazil, India, South Africa, and North America. The image alone cannot confirm a precise variety.