Bentonite / Marconite is a synthetic material manufactured specifically for earthing purposes.
Bentonite is a moisture retaining clay used as an earth electrode back-fill to help lower soil resistivity. The Bentonite clay is a sodium activated montmorillonite which when mixed with water swells to many times its original volume.
The Marconite compound then goes through a range of manufacturing processes and thermal treatments, designed to create a consistent, fit-for-purpose earthing compound.