Quick answer: Magnetite (Fe3O4) is the most magnetic naturally occurring mineral: a magnet sticks to it, and some pieces, called lodestones, are natural magnets themselves, which is how the first compasses were made. It is a major iron ore, black with a black streak, and about 5.5 to 6.5 on the Mohs scale.
Most minerals nicknamed after a property they merely resemble don’t actually have that property. Magnetite is the exception. It’s genuinely, physically magnetic, strongly enough that a natural magnetite specimen, a lodestone, can pick up iron filings and paperclips directly, no battery or coil required.
An iron oxide with an unusual electron arrangement
Magnetite is an iron oxide, Fe3O4, containing iron in two different oxidation states within the same crystal structure, and that specific arrangement is what produces its magnetism: the unpaired electrons around those iron atoms align their magnetic fields in the same direction throughout the crystal, rather than canceling each other out the way they do in most iron-bearing minerals, including the closely related hematite. It’s a genuine physical property, not a folk association, in the same category as quartz’s real piezoelectric effect, and one of the very few minerals that will visibly deflect a compass needle held nearby.
A compass before there were compasses
Naturally magnetized magnetite, lodestone, is widely credited as the material behind the earliest magnetic compasses, used for navigation in China as early as the Han dynasty, long before anyone understood the physics of why it worked. Today magnetite remains one of the most economically important iron ores mined worldwide, and its magnetic behavior is put to more precise use in geology: because magnetite crystals lock in the direction of Earth’s magnetic field at the moment they cool, geologists read ancient rock formations for frozen magnetite orientations to reconstruct how continents have drifted and rotated over hundreds of millions of years, a record written directly into the mineral itself.
Magnetite vs hematite: how to tell them apart
Magnetite and hematite are both iron oxides, both dark and heavy, and both around 5.5 to 6.5 on the Mohs scale, so they get mixed up constantly. Two quick tests settle it:
- Streak. Rub the specimen across the back of an unglazed tile. Magnetite leaves a black streak. Hematite leaves a reddish-brown streak, even when the specimen itself is black or mirror-silver.
- Magnet test. Magnetite pulls hard on an ordinary fridge magnet. Natural hematite is only weakly magnetic at best, and many pieces show no pull you can feel.
Crystal shape helps too. Magnetite tends to grow as sharp octahedrons (two pyramids joined at the base), while hematite grows as flat plates, glittering “specular” flakes, or rounded botryoidal masses.
Is “magnetic hematite” real?
Mostly no. The shiny silver magnetic beads and bracelets sold as “magnetic hematite” or “hematine” are almost always a manufactured ferrite ceramic, not natural hematite. If a polished “hematite” piece snaps hard onto a magnet, it is either synthetic or carries a lot of magnetite.
Lodestone: why only some magnetite is a natural magnet
Every piece of magnetite is attracted to a magnet, but very little of it is a magnet. Lodestone is magnetite that has been magnetized in the ground. The leading explanation is the huge, brief currents from lightning strikes. That’s why lodestone is found near the surface and is rare compared to ordinary magnetite.
Where you can find magnetite yourself
Magnetite is one of the easiest minerals to collect. The dark streaks of “black sand” on many beaches, including much of Long Island’s shoreline, are rich in magnetite grains, often mixed with tiny red garnets. Put a magnet in a zip-top bag and drag it through the sand, then turn the bag inside out over a dish to release what it caught. Inland, New York’s Adirondacks were once a major magnetite iron-mining district.
Hematite also turns up as an inclusion in other minerals. It’s what makes these hematite-included amethyst clusters red.
Keep reading: Can you find gemstones on Long Island beaches? Garnet sand, magnetite sand, and Montauk’s faintly radioactive black sand.
