Blue Burble

One morning many years ago, while walking down to the Wardtown swamp to do some Wood Duck hunting, I came across an area that the landowner had excavated out for construction of a spillway. The layers of earth were obviously undisturbed, and the lower layers consisted of a yellow clay, sand mix. I noticed an odd stone partially exposed on the side walls, and dug it out to examine. It was a blue round ball of what appeared to be glass, but how did it get to be more than 10 foot below the surface, in what seemed to be virgin soil? I stuck it in my pocket and went on my way to my brushed up duck blind.

Later that evening, I pulled it from my coat pocket and held it up to a light. It seemed to be glass. Having some old blue bottles around, I compared the colors, this was not the same color at all, as say the old Milk of Magnesia blue bottles. So I looked up blue earth glass, apparently this natural phenomenon exists, possible caused by lightning strikes? Anyway, I never had any expert opinions, but any thought or ideas what it could be are appreciated!