ירמיהו יז:א פירוש: רש״י

with a pen of iron This is an allegory. That is to say that it is deeply engraved, and cannot be erased. And the Midrash Aggadah explains: with a pen of iron, with a diamond point, preserved by Jeremiah, called ‘an iron wall,’ and by Ezekiel, to whom it was said (3:9): “As a diamond, harder than flint have I made your forehead.”

engraved Engraved like one who plows deeply.

on the tablet of their heart that they do not forget their idolatry.