The absolute permeability of a substance is less than that of free space. What can we say about it?

The susceptibility of substance is less than sodium.

The absolute permeability of substance is less than that of free space. This means that the substance has to be none other than a diamagnetic material. 

1. Sodium is paramagnetic. 0 < X_para < ε and -1 <= X_dia < 0. Thus, susceptibility of substance is less than sodium. ε is a small positive value.
2. The substance is NOT ferromagnetic. 
3. Lodestone is ferromagnetic.