From tsaphar (in the denominative sense (from tsippowr) of scratching); properly, a claw, i.e. (human) nail; also the point of a style (or pen, tipped with adamant) -- nail, point.
KJV: 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;
NASB: The sin of Judah is written down with an iron stylus; With a diamond point it is engraved upon the tablet of their heart And on the horns of their altars,