A few years back I was working in my shop when I spilled a box of screws. I picked up most by hand, but there were still many screws scattered about, so I grabbed a magnet and swept them up off the floor. But I noticed that there were also a few small rocks caught by the magnet. I recognized them as the paving gravel on my driveway, so I went outside and ran the magnet over a few piles of loose gravel. Sure enough, it caught quite a few little bits of gravel.
I thought that was curious, but left it at that. But a few weeks ago I ran across a mention of magnetite in mines in the area. So I decided to test the possibility that road gravel in the region contains magnetite. I stopped at a pullout a couple of miles from my home; here’s the result:
Magnetite is, obviously, an ore of iron; its chemical formula is Fe3O4.