hard as rock
- solidFirm matter state
- stoneSolid earth fragment
- rigidUnable to bend
- unyieldingRefusing to give way
- toughDurable and strong
granite · concrete · firm · dense · diamond · iron · boulder · immovable · stiff · unbreakable · petrified · marble · stubborn · inflexible · steel · frozen · compact · flinty
physical texture/consistency
hard as rocksoft as butter
soft as a cloud · soft as silk · squishy · mushy · spongy
difficulty/challenge
hard as rockeasy as pie
Extremely simple to accomplish
piece of cake · child's play · a breeze · effortless · a walk in the park