Most placer miners have voiced their opinions about pesky magnetite. And that’s understandable, because magnetite is the primary component of the ubiquitous, heavy, black sands that often clog sluice box riffles. But there’s another side to magnetite that deserves respect, for this iron-oxide mineral has profoundly influenced history, culture, industry, and science.

More than a half-billion tons of magnetite ore are currently mined worldwide each year as a source of iron. And geophysicists study magnetite grains in igneous rocks to learn about the ancient Earth’s magnetic fields and tectonic-plate movements.
Magnetite (iron oxide, Fe3O4) consists by weight of 72.36 percent iron and 27.64 percent oxygen. It crystallizes in the isometric system, usually as octahedrons, occasionally as dodecahedrons, and rarely as cubes. Opaque and with a submetallic-to-metallic luster, magnetite is black to dark-gray in color with an occasional hint of blue iridescence. Brittle and with a subconchoidal-to-uneven fracture, it has a Mohs hardness of 5.5-6.5 and a substantial specific gravity of 5.17.
Abundant and widely distributed, magnetite is present in most mineral environments. It occurs in particulate, crystalline, and massive forms and is a common accessory mineral in igneous, metamorphic, and sedimentary rocks. When magnetite weathers free from host rocks, its density enables it to concentrate gravitationally on beaches and in placer deposits.
Magnetite’s most notable physical property is its natural magnetism, which is by far the most intense of any natural material. Many minerals exhibit trace magnetism, but few have any significant level of magnetic susceptibility. On the magnetic-susceptibility scale, nonmagnetic minerals are rated at zero. Magnetite, the only mineral with obvious magnetism, is rated at 20. Next is chromite (iron chromium oxide, FeCr2O4), with a magnetic-susceptibility rating of 1.0.
Although “normal” magnetite is attracted to magnets, it does not attract bits of steel (or other bits of magnetite). Only lodestone, the relatively rare variety of “automagnetized” magnetite, has sufficient magnetism to attract steel. The word “lodestone” stems from the obsolete word “lode,” which meant “course.” When lodestone appeared in Middle English in the early 1500’s, it meant “leading stone” or “course stone,” alluding to its use in compasses.
