the quality of being difficult to do; "he assigned a series of problems of increasing hardness"; "the ruggedness of his exams caused half the class to fail"
the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale
the quality of being solid and reliable financially or factually or morally; "the solidity of the evidence worked in his favor"; "the solidness of her faith gave her enduring hope"