Vermiculite can be found in potting soil or purchased by itself in four different sizes for gardening with vermiculite. Germinate seeds using the smallest size of vermiculite as a growing medium and the largest size for improved soil aeration. Vermiculite is the name of a group of hydrated laminar minerals (aluminum-iron magnesium silicates) which look like mica. Horticultural vermiculite is processed with massive heat that expands it into accordion shaped pellets composed of multiple layers of thin plates. It will not rot, deteriorate, or mold and is enduring, odorless, non-toxic and sterile.