hard as nails.

[ImLostAgain]

Why hard as nails? Why not hard as rock, diamond, granite or some such?

[Clanad]

One of my desktop sources says "...Unyielding, callous, unsympathetic, as in Don't ask her for a contribution, she's hard as nails. This expression has replaced the 14th-century simile hard as flint stone and presumably alludes to the rigidity of nails..."

However, another source places the first written useage as " 1838, Twist, chapter ix, "Hard" replied the Dodger, "as nails", added Charley Bates..."

[ImLostAgain]

Cheers Clanad. Good answer. Interesting about the 'Twist' connection.
As an afterthought I remember a little old guy in my then local who wouldn't have weighed six and a half stone wet through who used to often say, " I'm as hard as nails me, easily hammered".