rock hard's Usage Examples:

NME wrote, Souljacker's songs rock harder than most of E's nu metal enemies.

fascial dystrophy") is a cutaneous condition characterized by ‘rock hard’ induration, thickening of the skin and subcutaneous tissues, limited joint mobility.

Synonyms:

serious, tough, thorny, problematical, demanding, rugged, difficult, touchy, challenging, knotty, rocky, embarrassing, tricky, arduous, difficultness, hard-fought, problematic, vexed, nasty, trying, unenviable, awkward, troublesome, fractious, baffling, effortful, elusive, sticky, herculean, tight, difficulty, tall, ticklish, ambitious, catchy, delicate, rough,

Antonyms:

ease, effortless, easy, undemanding, frivolity,