The piece above seems suggestive to me—that one flake came off right where you might start if you were going to use it for something. And it’s the kind of shape you might choose for a digging tool. It was worn all around the edge—possibly by the waves, of course. Still, it’s interesting to me because I would not necessarily have seen this rock as knappable if I’d seen it when it was whole.
This is the closest I’ve gotten to controlled knapping—the flake on the bottom is nicely conchoidal (shell-shaped). It’s teaching me a lot about the rocks around here, and I’m slowly developing ideas about what I should be looking for. When I started out, I thought that I should look for smallish pieces if I wanted to make a smallish point. But what I’ve noticed, from watching a lot of knapping videos (check out Jack Crafty’s channel on youtube) that when a skilled person finishes a preform, they are left with a thin blade of very homogenous material. That is, the material doesn’t have internal cracks and crumbliness. A lot of times, especially on the coastline the rocks are banged about a lot, so unless you find a freshly-fractured piece or start with a larger piece, you find that there are faults that extend all the way through.
The other thing is that the rocks around here tend not to be perfectly glassy like flint or obsidian—even the nicer ones tend to want to break along one of two perpendicular planes. So you kind of have to try to get a piece where the midline is at 45 degrees from those planes—which again, with this material, tends to mean starting with a larger initial piece and reducing it to see where the nice even interior volume is inside it.