set in stone vs solid as a rock

set in stone

adj

  • Permanent; certain; firm. 

verb

  • To render something permanent, certain, or firm. 

solid as a rock

adj

  • Very reliable and dependable 

  • Extremely thick and heavy, so as to make it impossible to move. 

How often have the words set in stone and solid as a rock occurred in a corpus of books? (source: Google Ngram Viewer )