Akadama is the very same and (perhaps once) mysterious medium you will see exotic succulent and cacti cultivators including our succulent growers using. 
 
Succulents require extremely well-draining medium to thrive. Not only is it important to not overwater, the correct growing medium can significantly reduce the risk of root rot. 
 
Akadama is mainly from Japan and is formed as a result of the eruptions of Mt. Fuji on Honshu Island across centuries. They're highly prized among Succulent and Bonsai growers worldwide mainly for its porosity. It is able to retain water and nutrients within its microscopic pores and cavities while draining off any excess water. This free draining open structure of the Akadama prevents water-logging and allows aeration and effective plant root development. Akadama also darkens when moistened, thereby helping the caretaker know when to water the plant. 
 
Generally, Fine-Grain Akadama is used for smaller succulents and Medium-Grain Akadama for larger ones.