October 4, 2006:
Iron filings.
The play field is large and square, as big as a city block from one end to the next.
The sand is coarse, and it's multicolored when you look closely.
There's a purification process. The first pass picks up as much sand as iron. A pass through those: the results are more clean. A final pass leaves almost all black. On a paper towel from the boys bathroom.
Your magnet is round, a ring. It looks mechanical, a fitted part of a larger apparatus. Maybe it was intended to belong to an engine of some kind.
You store your purified iron in a small glass vitamin bottle with a stopper. Chocks? Could be.
Small things are as interesting as large.