Expanded Shale or Haydite. 4 Quart bag.Here's a product with basically everything you need for your Bonsai. It helps your Bonsai soil stay loose by increasing the porosity of the soil. It conserves and holds water so you don't have to water as often even with our very porous soils and keeps you from the other extreme of over watering. It enhances root development and growth.
This is mixed grain sizes, meaning small and medium grain sizes. A US Quarter was placed in the picture to give you an idea of scale. Available in 2, 4 and 8 quart bags.