Horticultural charcoal for terrariums. A thin layer placed under the substrate helps keep the water in the base fresh, which matters in a closed glass container.
It works alongside the clay drainage layer and the substrate mix, so the three form a simple base for a closed jar. Place it after the drainage layer and before the substrate, so the water passes through it on its way down. Keep the rest of the pack sealed between builds so the charcoal stays dry.