Vermiculite can also be used for storing bulbs and winter vegetables, and as a base for floral arrangements. Widely used soil and potting mix amendment and a medium for starting seed and rooting cuttings. Useful for aeration, water retention and improving nutrient exchange. Made from mica rock which is expanded under pressure and heat. It is also used as a potting soil ingredient and as a medium for starting seedlings and rooting cuttings. When vermiculite ore is heated to almost 2,000 degrees F., moisture within it turns to steam and pops the granules to many times their original size. This rapid expansion leaves countless tiny air cells within the mineral. Vermiculite can hold several times its own weight in water but even when the medium is thoroughly wet, ample air circulates about plant roots, helping to avoid damping-off and other diseases. There is some concern about breathing this material.