Appearance:
* As hard as a diamond
* As rough as sandpaper
* As smooth as polished marble
* As gray as a storm cloud
* As red as a sunset
Texture:
* As cold as ice
* As porous as a sponge
* As heavy as lead
* As brittle as glass
Nature:
* As immovable as a mountain
* As ancient as the earth
* As silent as a tomb
You can choose the simile that best fits the context of your writing. For example, if you are describing a smooth, round rock, "as smooth as polished marble" would be a good choice.