Iron filings.
I have a small black rectangular magnet from I don't know where. And a coarse brown paper hand towel from the restroom. I work the magnet carefully through playground sand, back and forth, slowly, from my tiny staked claim next to the high chainlink schoolyard fence, until it becomes coated in black filings which seem almost fuzzy while attached. Then I scrape the filings onto the paper towel for further processing.
Was this a class assignment? Probably. It's difficult to imagine what other impetus there might have been for something so unlikely. Whatever, I was determined that my filings would be the purest in the land. From the paper I refined them, running the magnet through them again and again to capture the iron while leaving sandy dross behind. I purified them until they were solid black, then poured them into a clear glass bottle which once had held Chocs children's vitamins. With a somewhat ornate glass-and-rubber stopper, like a 19th Century ink bottle.
I don't know what became of that bottle. Only that for a certain time of my life it and its contents were meaningful to me, in an early aesthetic sense of a job well done.