Goethite (likely as part of Limonite/Ironstone)

Hardness: 5-5.5 (Mohs), Color: Yellow-brown to dark brown, often with iridescent blue or purple tarnish. Luster: Submetallic to earthy. Crystal structure: Orthorhombic, but often found in botryoidal, stalactitic, or earthy masses, less commonly in distinct crystals. Streak: Yellowish-brown. Specific Gravity: 3.3-4.3.

Goethite commonly forms as a secondary mineral through the weathering of iron-rich minerals. It can be found in a wide range of geological environments, including gossans (oxidized caps of sulfide ore deposits), bog iron deposits, and as a precipitate in hot springs. Its formation is typically ongoing through geological time as long as iron-bearing rocks are exposed to oxygen and water.

Goethite, particularly when abundant in ironstone or limonite deposits, serves as an important iron ore. It has also been used as a pigment (ochre) since ancient times. In its well-formed or iridescent varieties, it is sought after by mineral collectors.

Goethite is a major component of rust and limonite, which is a general term for hydrated iron oxides lacking distinct crystal form. It is one of the most common iron minerals and has been used as a pigment since prehistoric times. The famous 'Rainbow Goethite' from Negaunee, Michigan, displays a strong iridescence due to microscopic layers in its structure.

In the field, goethite is often identified by its characteristic yellowish-brown to dark brown color, earthy or botryoidal habit, and relatively low hardness (easily scratched by a knife). It can often be found in soils rich in iron, near areas of historical mining, or in sedimentary environments where iron has precipitated. The specimen in the image strongly suggests goethite or limonite due to its rusty orange-brown color and earthy appearance, typical of weathered iron minerals. It's commonly found in iron ore deposits worldwide, including the Lake Superior region of the United States, various European localities, and Australia.