Did not mean to dirstract from the thread with a mis-ID of a fulgerite. I did not know about fulgertites to recently, so perhaps this is a better picture:
looking at the OPs pics, who was told these were lightning strikes in sand, to me they look less likely to be created by lightning than the stone I posted earlier. I guess my question is, wouldn't these "Lightning Stones" be branched like the photo above, and not like a piece of man made glass broken off and exposed to the elements a bit?
In case I'm confused, the pic above is a bit to transparent to be any Chalcedony that I've seen. The little I've found is White and opaque with a little bit of rust staining.
The cracks in the pavement/sidewalk and the joints of paving bricks have been very good to me.
If you pick the right spot you can make several hundred dollars a day with nothing but a brush and a wire.
Techniques for urban prospecting are tricky and a lot of fun. Once you figure it out you can really haul in the goodies.
Hint - Forget the metal detector. Diamonds don't beep.