Charles Ellicott Commentary Jeremiah 17:1

"The sin of Judah is written with a pen of iron, [and] with the point of a diamond: it is graven upon the tablet of their heart, and upon the horns of your altars;" — Jeremiah 17:1 (ASV)

A pen of iron. — That is, a stylus, or graving tool, as in Job 19:24, chiefly used for engraving in stone or metal. In Psalms 45:1, it seems to have been used of the instrument with which the scribe wrote on his tablets.

With the point of a diamond. — The word expresses the idea of the hardness rather than the brilliancy of the diamond, and is rendered “adamant” in Ezekiel 3:9 and Zechariah 7:12. (For the diamond as a precious stone, a different word is used in Exodus 28:18.) Strictly speaking, it was applied only to the diamond-point set in iron used by engravers. Such instruments were known to the Romans (Pliny, Hist. Nat. xxxvii. 15) and may have been in use in Phoenicia or Palestine.

These words describe a note of infamy that could not be erased, and this was stamped upon the tablets of the heart (compare 2 Corinthians 3:3) and blazoned upon the “horns of the altars” of their false worship, or of the true worship of Jehovah which they had polluted and rendered false. The plural “altars” probably points to the former.