The words "stubborn faces" refer to the way they act on the outside, and the words "hard hearts" refer to the way they think and feel. Together they emphasize that the people of Israel would not change how they lived in order to obey God. (See: [[rc://en/ta/man/translate/figs-doublet]])
Rocks never change and become soft, and these people never change and become sorry when they do evil things. (See: [[rc://en/ta/man/translate/figs-metaphor]])