Jeremiah 17:1

חַטַּאת יְהוּדָה כְּתוּבָה בְּעֵט בַּרְזֶל בְּצִפֹּרֶן שָׁמִיר חֲרוּשָׁה עַל לוּחַ לִבָּם וּלְקַרְנוֹת מִזְבְּחוֹתֵיכֶם

The sin of Judah is written with a pen of iron and with the point of a diamond. It is engraved upon the tablet of their heart and upon the horns of your altars.

The Metzudas David explains that this verse continues from the end of the previous chapter, where it tells us that the nations of the world will abandon idolatry and come to recognize G-d. Nevertheless, the sin of Judah will already have become indelibly imprinted upon the people, as if carved there by the hardest implements imaginable.