It has taken me a lot of looking and cutting to start to figure it out. You have a minor advantage in that you are not entirely exploring new territory. I think my one cast of pile shows how many stone I've turned. The one in the picture represents about 1/3 of my total off casts. I have not found a fool proof method of finding rock in the field that will be free of inclusions. The very small inclusions sometimes don't show until lapping has started. I am starting to get into some predictable spots just by narrowing my sampling to smaller and smaller areas.