Limonite

Limonite, or BROWN IRON ORE, hydrous ferric oxide. This mineral occurs most frequently in the form of fibrous aggregates, or earthy and amorphous masses, and never in that of definite crystals. It has a hardness of 5.5, a specific gravity of 3.3–3.9, and a yellowish-brown streak. It is the yellow colouring matter of chalybeate springs. Rocks containing iron are often stained brown or yellow from the conversion of the iron into limonite.