Magnetite is one of the two most important iron ores, the other being hematite. Although hematite is more abundant world-wide, magnetite is more widely used in North America. Some of the notable iron ranges in the U.S. include the Mesabi Range in Minnesota and the Marquette and Menominee Ranges of Michigan. Magnetite is a black, opaque, magnetic mineral, and its spinal crystal structure contains both the ferrous (Fe+2) and ferric (Fe+3) forms of iron ions, and exhibits a ferromagnetic property. Magnetite ore is upgraded by magnetic separation from a low-grade taconite ore. This book discusses the synthesis, uses and biological occurrences of magnetite.