HEy John,
The BIG obsidian blades were usually wealth/status symbols for the elite. They were used by northern CA tribes like the Hupa, Karok, etc. They are normally known as Dance knives because they were used in ceremonies. They are usually a little narrower in the middle of the blade and widen a bit toward each tip. I've seen some pics of some that were 9-12" long, although I saw a photo of one that was about 22" long. That's the longest one I've ever seen. I've seen some that are solid black obsidian and I've seen some that were banded orange and black. Steve Allely knows all about them (he's replicated a few for collectors) but he's not on PA. Emory Coons and Jim Winn would probably know more about them than me, so if you can get in touch with them they might be able to divulge more info. hope that helps.