* As hard as a nail: This is the most straightforward and common simile.
* As unyielding as a hammered nail: Emphasizes the stubbornness and refusal to bend.
* As sharp as a tack: Highlights the piercing or pointed nature of the hardness.
* As solid as a nail in concrete: Implies an unshakeable and immovable strength.
* As impenetrable as a nail in a wooden plank: Focuses on the resistance to penetration.
The best simile will depend on the specific context and what aspect of hardness you want to emphasize.