Exodus 39:11

Overview - Exodus 39
The cloths of service and holy garments.
The ephod.
The breast-plate.
22 The robe of the ephod.
27 The coats, mitre, and girdle of fine linen.
30 The plate of the holy crown.
32 All is finished, reviewed, and approved by Moses.
Treasury of Scripture Knowledge

Exodus 39:11  (King James Version)
And the second row, an emerald, a sapphire, and a diamond.

a diamond
28:18 Ezekiel 28:13
The word {yahalom} may mean the diamond, from {halam,} to beat, smite, because of its extreme hardness, by which it will beat to pieces the other stones. It is a fine pellucid substance never debased with any admixture of other matter; susceptible of elegant tinges from metal-line particles; giving fire with steel; not fermenting with acid menstrua; scarcely calcinable by any degree of fire; and of one simple and permanent appearance in all lights. It is the hardest and most valuable of all gems; when pure, perfectly clear and pellucid as the clearest water, and eminently distinguished from all other substances by its vivid splendour, and the brightness of its reflections.