Vermiculite is a micaceous mineral that expands when heated to 2000°F. It is inert, disease-free and has excellent water holding and cation exchange capacity. It is available in several horticultural sizes, the smallest size used for seed germination. Its major drawback is that it can compress over time losing its structure.
Make sure you use horticultural grades because building construction grades have been treated with materials that can be toxic to plants.