I personally don’t believe there’s much difference in quality between English black flints, amber French flints, and so on. More depends on shape and sharpness, and the lock. It’s impossible to do an actual test with various knapped flints from different sources, because no 2 knapped flints are identical in shape and dimensions. And that’s what’s required for real head to head tests. As someone who knaps flints, I normally sort them out to be used in a particular gun with a particular lock. Some locks will destroy a thin, acute angled flint and others will give 25 or more firings with spectacular sparking. Some will spark well with flints that appear a little blunt in edge angle, and others not. Shape is what matters most.