People's Dictionary of the Bible
Adamant. This word is found twice in our version, Ezekiel 3:9; Zechariah 7:12, in both eases used metaphorically to signify firmness of character and purpose. The original word occurs again in Jeremiah 17:1, where it is translated "diamond," with which the writer's pen is said to be pointed. The term must signify some exceedingly hard stone; and diamond is the hardest we know.
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Rice, Edwin Wilbur, DD. Entry for 'Adamant'. People's Dictionary of the Bible. https://www.studylight.org/dictionaries/eng/rpd/a/adamant.html. 1893.