Vermiculite is a naturally occurring mineral (magnesium-aluminum-iron silicate) that is mined in various countries around the world. Lightweight, non-toxic and sterile and does not deteriorate over time. It has a neutral pH. For use in horticulture, it’s sold as lightweight, absorbent brown-gold flakes. Vermiculite can absorb three or four times its weight in water and also attracts various plant nutrients such as potassium, magnesium and calcium. These qualities make it useful for propagation and for potting up plants that do best in moist compost, including house plants.