Boyd’s Bible Dictionary:
(diamond). The original is translated “adamant” (Ezek. 3:9; Zech. 7:12); and “diamond” (Jer. 17:1). Used metaphorically. [DIAMOND.]
Concise Bible Dictionary:
(Ezek. 3:9; Zech. 7:12). Though once translated diamond (Jer. 17:1), it is used symbolically of extreme hardness, “harder than a flint.” The word is translated “briars” and signifies any sharp point: hence a diamond point, or anything hard.