There's a lot of variation within classifications (flint, chert, jasper, etc) and within one stone. If you are interested in making gunflints from your local stone, you can often find something that can suffice.
I agree that any form of glass (like obsidian) will not work. It's good to keep in mind that the flintlock was designed to work with....flint.
There's a lot of technical stuff to consider, like in any field, if you wish. For example we don't have any flint in America accoring to the rigorous definition of flint as only found in chalk beds. So most of what we call flint in America is tedchnically called chert, but that does not make any difference. I have made gunflints from jasper, chert and what would be called agate by most people.
Certainly Native Americans made projectile points and tools from whatever was available, and when not available, used bone, ivory, antler, etc. But the availability of good working "flint" (chert in many cases, obsidian etc in others) probably influenced where folks lived and with whom they traded.
I wrote an article a few years back for On the Trail magazine on how to find flint and make gunflints with simple tools. There was a good article in Muzzle Blasts some time ago on this also and there are a couple resources on the web.
If you already know how to knap rock by percussion it's pretty easy. I was self taught and so developed my own approach and tool kit.
Much will depend on the rock. From a pure homogeneous nodule of English or French etc flint the size of a football I might get 6 or 8 dozen flints if things went well and I didn't make many false moves. From a typical heterogeneous piece of Missouri chert with a thick rind of limestone and transitional rock surrounding a much smaller core of good knappable "flint" that has a bunch of inclusions and some internal cracks, maybe 1-2 dozen.
