Some " flints" are Manufactured- made of agate that is cut. When the cutter cuts the stone, he doesn't attempt to take into consideration the Natural fracture lines of the stone. Instead, he just cuts the rock in any direction that suits his purpose.( making as many "flints" from a rock as possible.)
Knapped flints are PRODUCED by taking into consideration their natural fracture lines, using percussion techniques to produce a gunflint. They are always a better flint to choose than a cut "flint".
The Tom Fuller flints from England are a standard for this industry, and Rich Pierce's knapped flint using flint from the Missouri deposits, are also good flints. There are other flints people on this forum make using a variety of materials, like Jasper, that also perform well. They are generally available for sale commercially. Then there is the Yellow " French-Amber" flints from France. Some are good; others aren't. I can't recommend them to shooters for that reason, when a good English flint, or a flint made by Rich Pierce is available at a reasonable price.
If you are new to shooting flintlocks, just Mounting the flint properly in the jaws of your **** may require instruction and coaching from an experienced flintlock shooter. If the flint is NOT mounted correctly, you may have trouble getting sparks from it.
Paul