I live within 4 hours of Friendship, so I buy my flints over at the Nationals, when I can hand pick out the flints I want, and avoid those with Humps. I have received a dozen flints from our Rich Pierce, but have not "shot them". I was given some of the cut Agate "flints", and they are terrible, for the reasons I stated in the earlier post.
D.C. Waldorf has a short "booklet", titled, " The Art of Flint Knapping". Chapter 4 is titled, " Elementary Flint Knapping, and discusses the "Hertzian cone"( approx. 100 degrees) involved in most if not all rocks made of silicon, or silicates.
He rightfully reminds us all of what a piece of glass looks like when struck with a BB. it has a smaller than BB diameter hole on the impact side, and a large, 100 degree cone through the glass from the entrance to the other side of the glass.
I suppose there is someone out there who has not seen a piece of glass shot with a BB, but I would like to think that group doesn't include many active shooters!
Knapping Takes advantage of this natural line of fracture, in flint, to form a gun flint. A tuned flintlock will take advantage of this line of fracture, to both cut steel, and then flake off a NEW edge on the flint for the next strike.
Agates, altho silicates,when cut, rather than knapped, are cut across these natural fracture lines within the stone, so that the cut edge works against making sparks, and knapping a new edge. A cut stone simply is an artificial "stone" that usually will not act like a natural stone acts when struck.
Once in awhile, I meet someone who had a cut agate flint that worked well for some time, and he got lots of sparks in his lock. But, even those rare shooters admit that they haven't had the same experience with other cut agate flints.
If the reader is one of the " Lucky " shooters who has had a good cut agate flint that sparked well, then you know what I mean. Of the half dozen I own, only one gave a few more sparks than the rest, and they are throwing only one or two sparks per strike. The Agate edges seem to just crumble, rather than flake.