Sounds like obsidian to me... part of nappin is to know were to hit it , when to hit it , and why ya hit it were ya hit it when ya hit it there.. ya gatta set up a platform before you strike the rock, failer to have the proper platform set up will result in small flaks or a crumbled edge.
Obsidian comes in many forms, sometimes black , sometime orange , sometime it has all the colores of the rainbow in it, and evon gold sometimes, all the obsidian I've ever sen ya can see through the thin edges except for dasite witch is a gray type of obsidian.
My advice is to watch some videos on flint napping and read some books on the subject before you go smashing up all yer rock. I'm not tryin to be rude all nappers start somewere and I'd hate to see you wast your rock.
Flintnapping takes alot of time frustration, practice a little knowhow and some blood to get the hang of it.. just stick in there and you will be reworded with a awsome new skill. Good luck with your new hobby, Leapingbare.