Goethite

Goethite is one of the most common iron-bearing minerals on Earth's surface, forming wherever iron-rich rocks weather in the presence of water and oxygen. It's a primary component of rust and the main mineral in limonite (the earthy brown material that stains so many rocks).

Well-crystallized goethite forms prismatic or acicular (needle-like) crystals with brilliant adamantine luster. Botryoidal (grape-like) goethite masses can develop stunning iridescent surfaces where thin-film interference creates rainbow colors on the dark brown base. This iridescent variety is sometimes called 'rainbow goethite.'

The mineral was named after Johann Wolfgang von Goethe, the German poet, novelist, and statesman who was also an avid mineral collector and amateur geologist. Goethe studied color theory and mineralogy extensively, making the naming particularly appropriate for a mineral that displays such varied visual effects.

Goethite is identified by its yellow-brown streak (diagnostic, distinguishing it from hematite's red streak), brown to dark brown color, and association with weathered iron-bearing rocks. Prismatic crystals have a bright adamantine luster, while massive forms are earthy.

Distinguish from hematite (red streak vs. goethite's yellow-brown), limonite (an aggregate containing goethite rather than a specific mineral), and magnetite (magnetic, black streak). The streak test is the single most useful identification method.