as hard as iron/marble/stone/nails

There's no perfect "the" answer. "As hard as..." creates a simile to describe how hard something is, physically or figuratively. Jura, Jennifer and Roi have given really good examples. It just gives you an idea. It's also possible to say "hard as" and leave the comparison unsaid. So the idea is that something is either *very* difficult, or *very* solid. This is a more slangy usage of the comparison. "That exam was hard as!" ='difficult as...??' It doesn't matter, it's clear enough that the exam was very difficult. "My new boss is as hard as." ='as stubborn as...??' Again, you already understand not to mess around with your boss "This toffee is as hard as." ='as solid as...??' But we already understand you can break your teeth on it. But it's still very worth learning the common associations for "as hard as". :)

as dick ， as cock，as nut ，as etc.

nuts cowboy...