Packed gravel, especially with a little clay is horrible to work with especially if you are clad hunting. That said, I was at the county fairgrounds one time on loose white gravel. Couldn't see a dime on the surface, the blending light patterns so difficult to distinguish, but it was where the dime toss tent had been. Had our older Lil' Lowbatts along and she stayed face just above the coil and I would say "there" and she'd poke a bit, we got almost 20 bucks in dimes in less than half an hour. All from one little area. But I was using the kid's fingers to do the pickin'. Don't do that much anymore, darn child labor laws....