Chemical Formula: FeO(OH)
Color: Mix of black, brown, and yellowish-brown
Importance: The main use of goethite is as a source of iron. Iron from goethite is used to make all sorts of metal alloys, like steel. The alloys made from the iron can be used in nearly any application. Iron from goethite is also used to make magnets.
Other Interesting Information: Goethite is known to form in botryoidal growths, which means that is forms in round shapes that look like bubbles. This makes it very easy to identify.
My Take: Personally, I love goethite. Not only does it form in unique shapes, but it also looks awesome. It has one color on the inside and another on the outside!
For More Information: http://www.minerals.net/mineral/goethite.aspx