What is a "flint?"
In verse 21, Nephi states that Laman and Lemuel had "hardened their hearts against (the Lord), that they had become like unto a flint." The Noah Webster 1828 Dictionary defines flint:
"1. In natural history, a sub-species of quartz, of a yellowish or bluish gray, or grayish black color. It is amorphous, interspersed in other stones, or in nodules or rounded lumps. Its surface is generally uneven, and covered with a rind or crust, either calcarious or argillaceous. It is very hard, strikes fire with steel, and is an ingredient in glass.
2. A piece of the above described stone used in firearms to strike fire.
3. Any thing proverbially hard; as a heart of flint.