Now, I realize that there is a way to easily detach the two materials. The clay just has to be dry, and then any blow with a hammer will split the clod exactly where the rocks are. There is still a good deal of work involved, because the rocks have to be picked out by hand, but it is work that can be done whenever I have a few minutes to spare. Instead of having to invest a few hours at a time to process the wet clay, I can pick over a couple handfuls when I feel like it.
Luckily the flint pieces are white, because the clods of dry clay and the stones are about the same hardness when touched. I can tell the difference by the colour and sometimes the geometry- the flint pieces have more thin edges.