Micrometeorites.

Micrometeorites are tiny meteorites that constantly bombard Earth. While tons of this material falls to Earth every year, collecting it can be a bit complicated.

I've actually collected some of them using a different, slightly simpler method. I wrapped a strong magnet in clean plastic wrap and ran it over the ground in an area with no vegetation. After carefully removing the plastic from the magnet without losing the material stuck to it, I looked at the specks under a microscope and saw a few of the pitted spheres described in the links above. I have no life.