It is non-metallic, contains iron and magnesium, when heated it swells and expands into light worm-like pieces of vermiculite which is used as a medium for planting seeds. Thanks!
I have a friend of mine just grows them inbetween paper towels, every layer is more seeds, when he wants them or they are ready he pulls out a wad of damp paper towels from this plastic container scoops out his micros, throws more seed in those paper towels or new ones and into another tray it goes with a spritz of hydrogen peroxide, he never seems to be out and they are fresh and clean.
