It does look like it could be obsidian, but it could also be slag, which is an industrial byproduct. Especially if there were ever any metal smelting operations/blast furnaces in the area. Some of it can look quite glassy.
That is very cool. I would love to find something like that in my backyard. Have you checked on line to see what minerals are indiginous
in your area. Don't get rid of it!