@Maddog - Nice point - buck rogersish or not - I would have to say that your material however is nice clear obsidian and not micah. Mica is REALLY thin layers packed together and would not carry a flake and would crumble under pressure flaking. there are two types I am aware of, Mica which was used as windows in early ovens and Biotite micah which is darker in color and is a primary ingredient in Granite (roundish dark crystaline structures). Yosemite has lots of granite - unfortunately it is no good for arrowheads.