Hematite & Goethite

Inside Hematite & Goethite

The rainbow sheen here is the same physics as an oil slick — not dye, not coating.

The story: Both minerals are weathering products of iron-bearing rock, and both were major players in the Great Oxidation Event roughly 2.4 billion years ago, when Earth’s oceans first filled with free oxygen and dissolved iron began rusting out of solution on a planetary scale. Where hematite and goethite grow intergrown in thin layers, they can produce the iridescent, rainbow-sheened surface seen on pieces like this one — a thin-film interference effect, the same optics as an oil slick on water.

Why collectors chase it: That metallic rainbow iridescence makes these among the most photogenic specimens in any mineral collection.

Care: Durable, but a real streak test (rubbing on unglazed porcelain) leaves a permanent red-brown mark — don’t test it on your own display tile.