sedimentary

Banded Iron Formation (BIF) / Tiger Iron

Magnetite / Hematite / Chert (SiO2 + Fe2O3/Fe3O4)

Hardness: 6.5-7 (Mohs). Color: Alternating bands of silver-grey (hematite/magnetite), red/brown (jasper/limonite), and golden-yellow (crocidolite). Luster: Metallic to dull or silky. Crystal Structure: Hexagonal/Trigonal. Cleavage: None (conchoidal fracture).

Hardness: 6
Luster: Metallic to dull or silky

Identified June 5, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed in ancient oceans during the Precambrian era (approx. 2.4 to 1.8 billion years ago) through the precipitation of iron oxides and silica as photosynthesis by cyanobacteria released oxygen into the water.

Uses & applications

Major source of the world's iron ore for the steel industry. High-quality varieties like Tiger Iron are used in jewelry (cabochons), carvings, and ornamental lapidary work.

Geological facts

BIFs are time capsules for the 'Great Oxidation Event'. These rocks formed when the Earth's atmosphere was first becoming oxygenated. Huge deposits are found in the Hamersley Range of Australia and the Lake Superior region.

Field identification & locations

Identify by its heavy weight (high specific gravity due to iron), magnetic properties if magnetite is present, and distinct alternating bands of sub-metallic and earthy layers. Found in ancient cratons.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite

sedimentary