Good for you. I was looking for Cassiterite in Md., and this came up. Do you have a microscope you could set up to look at it at 40-60 power? You might pick up the individual crystal structure, if there is. I don't know anything. I am fooling around with rocks to try and teach myself something about geology. Get a book buy Schmit(sp.?). An older paperback about Md. geology....has a picture of Sidling Hill on the cover...1993, me thinks. I just leant mine out. It is the one book you want to educate yourself about the area, in my opinion.
The rock/thing at hand...the little vesicles are from something, lava is light, slag has them, so does other volcanic stuff. The problem with rock identification is that you kinda hafta get a whole picture of everything at the same time to do it...I can not. That is why people concentrate on minerals, I think, not the varied, and complex causations, such, that caused them. Good luck, down near the Monocacy.