Verses with the word diadem in the Old Testament (4 verses):

Job 29:14
I put on righteousness, and it clothed me: my judgment was as a robe and a diadem.

Isaiah 28:5
In that day shall the LORD of hosts be for a crown of glory, and for a diadem of beauty, unto the residue of his people,

Isaiah 62:3
Thou shalt also be a crown of glory in the hand of the LORD, and a royal diadem in the hand of thy God.

Ezekiel 21:26
Thus saith the Lord GOD; Remove the diadem, and take off the crown: this shall not be the same: exalt him that is low, and abase him that is high.