Kitto's Popular Cyclopedia of Biblial Literature
The word thus rendered is, in Hebrew, Shamir. It occurs in Jeremiah 17:1; Ezekiel 3:9; Zechariah 7:12. The Septuagint in Jeremiah 17:1, and the Vulgate in all these passages, take it for the diamond. The signification of the word, 'a sharp point,' countenances this interpretation, the diamond being for its hardness used in perforating and cutting other minerals. Indeed, this use of the shamir is distinctly alluded to in Jeremiah 17:1, where the stylus pointed with it is distinguished from one of iron. The two other passages also favor this view by using it figuratively to express the hardness and obduracy of the Israelites. Our Authorized Version has 'diamond' in Jeremiah 17:1, and 'adamant' in the other texts: but in the original the word is the same in all. Bochart, however, rejects the usual explanation, and conceives it to mean 'emery.' This is a calcined iron mixed with siliceous earth, occurring in livid scales of such hardness that in ancient times, as at present, it was used for polishing and engraving precious stones, diamonds excepted. Rosenmüller urges in favor of this notion that if the Hebrews had been acquainted with the diamond, and with the manner of working it, we should doubtless have found it among the stones of the high-priest's breastplate; and that, as the shamir was not one of the stones thus employed, therefore it was not the diamond. But to this it may be answered, that it was perhaps not used because it could not be engraved on, or was possibly not introduced until a later period.
Kitto, John, ed. Entry for 'Adamant'. "Kitto's Popular Cyclopedia of Biblial Literature". https://www.studylight.org/encyclopedias/eng/kbe/a/adamant.html.