flint

This very hard mineral abounds in all the plains and valleys of the wilderness of the 40 years’ wanderings after the Exodus.

At that time the Lord said to Joshua, “Make flint knives for yourself, and circumcise the sons of Israel again the second time.” —Joshua 5:2 NKJV

Symbolic of hardness

…who brought water for you out of the flinty rock… —Deuteronomy 8:15 NKJV

…Therefore I have set My face like a flint… —Isaiah 50:7 KJV

…He made him draw honey from the rock, And oil from the flinty rock — Deuteronomy 32:13 NKJV

In the above verse, the use of the word flint means that the “Messiah would be firm and resolute amidst all contempt and scorn which he would meet; that he had made up his mind to endure it, and would not shrink from any kind or degree of suffering which would be necessary to accomplish the great work in which he was engaged.”

Similarly, God said to the prophet Ezekiel,

Behold, I have made your face strong against their faces, and your forehead strong against their foreheads. Like adamant stone, harder than flint, I have made your forehead; do not be afraid of them, nor be dismayed at their looks, though they are a rebellious house. —Ezek. 3:8-9 NKJV

Yes, they made their hearts like flint, refusing to hear the law and the words which the Lord of hosts had sent by His Spirit through the former prophets. Thus great wrath came from the Lord of hosts. —Zechariah 7:12 NKJV

The King James translation of Isaiah 5:28 uses the words “like a flint” in reference to horse hooves. The New King James Version says,

…Their horses’ hooves will seem like flint,
And their wheels like a whirlwind. —Isaiah 5:28 NKJV