--- I wrote this a while ago, but never got around to publishing it.
I think most outdoors nuts have heard of flint knapping.
When I was a kid I got into archery. Jess and I picked up the book "Naked into the wilderness" which had directions to make a bow and arrows starting from a dead deer, a tree, and some rocks. We also realized in reading this book that a huge portion of bush-craft is dependent on the hide and sinew from relatively large game. This inspired me to pick it up again with an eye towards getting good enough to hunt.
So, last weekend I was at the "earth dance" festival most proximal to San Francisco - and someone was teaching flint knapping! Awesome.
Flint Knapping, for those who don't know, is the process by which you turn a certain type of rock into a sharp cutting/piercing tool (such as an arrowhead).
The idea is simple. You strike a rock and it breaks. Certain types of rocks are very consistent, and don't contain structures that constrain the angles they will break at. Examples of this are cherts and obsidian. With these rocks exactly how you strike the rock, and what other forces you are putting on it at the time let you decide (largely) where it breaks. Thus, with repeated breaks you can slowly shape a tool.
This is one hell of an art. The guy who was teaching could fairly deterministically make an arrowhead in just a few minutes. It took me 4 attempts, and probably as many hours. The first 3 I cracked in half half-way through the process. And that's not counting the rocks I struck once or twice and gave up on.
Relevent facts are:
1) the rock breaks at ~120 angle from the angle at which you strike it
2) it's easier to transfer energy if the striking surface is rough, so the striker catches. Also, if the edge just cracks off it won't transfer energy deeper into the piece. So if you want to take a shard off that goes a long way across the piece, you have to strike a blunt face.
3) an arrowhead needs to be quit thin to slip between the two halves of a shaft for binding
4) making something thin is the hardest part, so that's your first priority. Shaping is easier
5) don't cut yourself! the edges are REALLY sharp
So, first, find your rock to make the arrowhead out of. You'll also want a rock to strike with, a round riverstone, sandstone rock about the size of your first works well. You can also use a lump of solid copper, or a chunk of bone.
That's basically it. With that in mind, you hold the rock with a piece of decently thick leather. Start with a larger piece and find a nice flat face to take a shard of, this shard will be the arrowhead. Next find a blunt face that points 120 degrees or so off from this. Take a rough rock and rub the second face to make a good striking surface. Place the first face against your thigh, on top of the leather. You'll want to be sitting on something, and not cross-legged, so your leg is solid. Strike the blunt face. It's easiest to strike with a wrist motion. You want a strike on the edge of the face, with a follow-through, you're looking to crack a long thin chunk off the bottom of the rock - not split it in half. Hit it lighter rather than heavier - you'd rather do nothing than split the entire rock in half.
This is one of 2 types of flaking. I just described strike-flaking. The other type is pressure flaking. It works exactly the same as strike-flaking, except that instead of striking, you just push really hard on the point you'd strike. This takes a lot of force, so it only works well once your piece is small, and you're taking off small pieces.
Hopefully you get a long thin slice off the back of the rock. If you're really lucky it'll be very nearly the shape of an arrowhead already. Most likely it will still require some shaping.
You may notice that you no longer have a good face to strike again (if you got it to break right). What you can do now is crunch the edges down, by just snapping then off, or by taking shallower flakes, until you have a better shape to flake against at all. You'll want to roughen the edges regularly as you go so you can get purchase for the flaking. Using this process slowly create the shape of an arrowhead.
Note that the hardest part is making it sufficiently thin. At the same time you don't want the whole thing *really* thin, or it'll just shatter. Because of this the most desirable break, what you're always trying to do, is get a long-thin flake off, since this lets you carefully control the resulting thickness.
That's really all there is too it! The rest is practice and skill. After about 4 hours, and shattering as many half-made arrowheads (and several more rocks earlier in the process), I got something that's pretty-much an arrowhead. As you can see I didn't quite finish flaking out notches to bind it to an arrow, but that wouldn't be that hard.