I wouldn't know a flint if it reached up and bit me so, it's probably not surprising that I haven't tried knapping one to make a "flint".
I've seen a lot of volcanic lava, quartz, granite, petrified wood and some "Apache tears" ( obsidian ). I recognize these.
I've also seen rocks and whole mountains plus my fireplace made out of sandstone.
I've also seen a whole bunch of red rocks, gray rocks, tan rocks, white rocks, black rocks and even some rocks with copper veins in them but I don't think any of these would made anything that would knap into something that would make a frizzen spark.
That's why I'm not a great believer in the old,
"You can just find flints all over the place.
Just reach down and grab some rocks to keep your flinter shooting. Flints are just laying around so you will never have to worry about having a flint for shooting your flintlock."
I suspect many of the people that say things like this don't know a flint from a piece of sandstone.