Vermiculite is a natural mineral that expands when heated. Raw vermiculite is a type of silicate mineral that looks like shiny black or brown flakes. Horticultural vermiculite is made by heating the raw vermiculite. As a result it expands up to 30 times its original size. It also takes on an accordion shape.
Vermiculite is used in the garden or potting soil to help aerate the soil and increase water and nutrient retention. Vermiculite is an excellent choice for amending the soil of moisture loving plants because of its water retention properties. It is also useful for encouraging faster root growth in cuttings. One of the benefits of horticultural vermiculite is that it doesn’t break down in the soil.