חַטַּ֣את יְהוּדָ֗ה כְּתוּבָ֛ה בְּעֵ֥ט בַּרְזֶ֖ל בְּצִפֹּ֣רֶן שָׁמִ֑יר חֲרוּשָׁה֙ עַל־ל֣וּחַ לִבָּ֔ם וּלְקַרְנ֖וֹת מִזְבְּחוֹתֵיכֶֽם׃
猶大的罪是用鐵筆、用金剛鑽的尖頭寫下的，深深刻在他們的心版上，也刻在你們祭壇的角上。
Rashi
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.”