I'm not sure where to post this, but I wanted to show this just because of the incredible nature of the phenomenon. It's just the dust from a slow running wet grindstone. I assume just vibration, but the more carefully you look the more unbelievable it becomes. As the grindstone ran, some of the water spilled over the edge of the reservoir carrying the grit. I put a HUGE magnet next to it and repeated the experiment. It made no difference. I also did it on a fiberglass tray. Same again. So the vibration factor would have been very different. The photos are not very good, but the more you look, the more complexity you can see.