Hard seeds are impervious to water. This is a type of dormancy known as Exogenous Physical dormancy.
Impermeability of the seed coat is due to a layer of macrosclereid cells.
In order to get these seeds to imbibe water, they must first have this outer layer of cells treated by the process of scarification.
This treatment involves physically disrupting the outer surface with an abrasive material like a file or chemically treated with concentrated sulfuric acid.