The sin of Judah is written with a pen of iron, and with the point of a diamond: it is graven upon the table of their heart, and upon the horns of your altars;
The Septuagint omit the first four verses, but other Greek versions have them.
The first of the four clauses relates to the third, the second to the fourth, by alternate parallelism. The sense is, they are, as keen after idols as if their propensity was "graven with an iron pen (Job 19:24) on their hearts;" or as if it were sactioned by a law 'inscribed with a diamond point' on their altars. The names of their gods used to be written on "the horns, of the altars" (Acts 17:23). As the clause "on their hearts" refers to their inward propensity, so "upon ... altars," the outward exhibition of it. Others refer "on the horns of ... altars" to their staining them with the blood of victims, in imitation of the Levitical precept (Exodus 29:12; Leviticus 4:7; Leviticus 4:18); but "written ... graven" would thus be inappropriate.
The sin of Judah written with a pen of iron, and with the point of a diamond: it is graven upon the table of their heart - which God intended to be inscribed very differently-namely, with His truths (Proverbs 3:3, "Write them (mercy and truth) upon the table of thine heart;" 2 Corinthians 3:3, "The letter of God, not written with ink, but with the Spirit of the living God: not in tables of stone (whereon the decalogue was, written), but the ink, but with the Spirit of the living God: not in tables of stone (whereon the decalogue was, written), but the fleshly tables of the heart").
Your - though "their" preceded, he directly addresses them, to charge the guilt home to them in particular: "your altars."