* As hard as rock: This is the most basic and common comparison, used for things that are very solid and difficult to break.
* As hard as steel: This simile emphasizes the strength and resilience of the thing being compared.
* As hard as diamond: This comparison emphasizes the extreme hardness and durability of the object.
* As hard as nails: This simile highlights the lack of flexibility and the firmness of the compared item.
* As hard as concrete: This comparison emphasizes the solidity and unyielding nature of the thing being compared.
You can also use "as hard as" to compare things to other hard objects, such as:
* As hard as a bone
* As hard as a brick
* As hard as a skull
The specific comparison you use will depend on the context and what you want to emphasize about the thing being compared.