Boyd’s Bible Dictionary:
(adamant). Pure crystallized carbon. Third stone in second row of high-priest’s breastplate (Ex. 28:18; Ezek. 28:13).
Concise Bible Dictionary:
The Hebrew word in Exodus 28:18; Exodus 39:11 and Ezekiel 28:13, is yahalom. It occurs only in these places, and cannot be identified: it is generally held not to be what is now known as the diamond. In Jeremiah 17:1 The word is shamir. This is translated “adamant” in Ezekiel 3:9. It is thought to be the corundum, a very hard stone, but being of different hues it has now various names.
Corundum (Var. Sapphire)
Diamond
Strong’s Dictionary of Hebrew Words:
Meaning:
from 1986 (in the sense of hardness); a precious stone, probably onyx
Jackson’s Dictionary of Scripture Proper Names:
he will smite down