Tiger iron is a striking natural stone made up of layered tiger’s eye, hematite, and red jasper. Its bold bands of metallic silver, deep red, gold, and brown were formed through ancient geological processes that compressed and transformed iron-rich sediments over immense spans of time. When cut and polished, tiger iron can reveal dramatic chatoyance—the shimmering, cat’s-eye effect associated with tiger’s eye—alongside the rich contrast of jasper and hematite. Each piece is naturally unique, making tiger iron especially popular for display specimens, cabochons, jewelry, and polished slabs.

The rock is essentially a piece of Earth’s early atmospheric history. Much of the well-known Australian tiger iron comes from ancient banded iron formations (BIFs) in Western Australia. Those iron-rich sediments were deposited on ancient seafloors roughly 2.4–2.7 billion years ago, although some tiger-iron-bearing formations can be even older. The Australian Museum describes similar Pilbara BIF material as about 2.5 billion years old.

The connection to the atmosphere comes from the iron bands. Early Earth’s oceans contained enormous amounts of dissolved iron because there was very little free oxygen available to oxidize it. As photosynthetic microorganisms began producing oxygen, that oxygen reacted with dissolved iron, converting it into insoluble iron oxides that settled onto the seafloor. Repeated changes in ocean chemistry produced the alternating iron-rich and silica-rich layers characteristic of BIFs.

Because of this, geologists study BIFs using things such as iron isotopes, sulfur isotopes, trace elements, mineral chemistry, and the timing of the layers to reconstruct how oxygen levels changed in Earth's oceans and atmosphere. BIFs are particularly important for understanding the transition surrounding the Great Oxidation Event about 2.4 billion years ago, when oxygen began becoming a persistent component of Earth's atmosphere.