Sorry, Ann, I just couldn't resist. Try Bug-B-Gone. Maybe you have aphids causing the crab apples in the orchard to turn yellow.
Seriously, can you give us any idea what kind of stone Crab Orchard is? There are so many local variations on the names of rock that few will understand the question. Some basic stones types are sandstone, limestone, shale, igneous (like granite). If you have a college near you with a geology department, you might ask them for some direction.
One thought I did have is since you got a reaction to acid, wonder what the reaction would be to a base. If you have a piece that is out of the way or extra and yellow, what happens if you put household bleach on it?