Easton's Bible Dictionary (1897)
(Heb. shamir), Ezek. 3:9. The Greek word adamas means diamond. This stone is not referred to, but corundum or some kind of hard steel. It is an emblem of firmness in resisting adversaries of the truth (Zech. 7:12), and of hard-heartedness against the truth (Jer. 17:1).
Smith's Bible Dictionary (1863)
” In these three passages the word is the representative of some stone of excessive hardness, and is used metaphorically. It is very probable that by Shamir is intended emery, a variety of corundum, a mineral inferior, only to the diamond in hardness.
Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
AD'AMANT. Eze. 3 : 9. This word means the unconquerable, and denotes some very hard stone. The same substance in Jer 17:1 is called diamond, which it cannot be, for the Hebrew name there used is never mentioned with precious stones. Probably it was the mineral emery, one of the hardest of rocks.