Flint is mentioned in The Bible both literally, as used by people during Bible History, and figuratively, in terms of its hardness, as an analogy for either righteous determination or foolish stubbornness. Some examples of both:
"Then Zipporah took a flint and cut off her son's foreskin, and touched Moses' feet with it, and said, "Surely you are a bridegroom of blood to me!" So He let him alone. Then it was that she said, "You are a bridegroom of blood," because of the circumcision." (Exodus 4:25-26 RSV)
"At that time The Lord said to Joshua, "Make flint knives and circumcise the people of Israel again the second time." So Joshua made flint knives, and circumcised the people of Israel at Gibeath-haaraloth. And this is the reason why Joshua circumcised them: all the males of the people who came out of Egypt, all the men of war, had died on the way in the wilderness after they had come out of Egypt. Though all the people who came out had been circumcised, yet all the people that were born on the way in the wilderness after they had come out of Egypt had not been circumcised." (Joshua 5:2-5 RSV)
"For the Lord God helps me; therefore I have not been confounded; therefore I have set my face like a flint, and I know that I shall not be put to shame; He who vindicates me is near." (Isaiah 50:7-8 RSV)
Fact Finder: Is a very hard mineral such as flint (NASB, NIV) or adamant (RSV, KJV) used as an analogy for the attitude of someone who refuses to repent?
Zechariah 7:12