Mother Nature's glass, obsidian is a joy to knap and gives up its flakes readily. It is very brittle though and care must be taken when using percussion. Our obsidian is normally a natural clear black, sometimes highlighted with reddish streaks. Our obsidian spalls are on the small side more for arrow points than knives. (Although you could make a knife from some of the larger pieces.) They can vary from a couple inches to over four inches. They're pre-bagged and sold by the pound, so expect a good mix of the various sizes.
If you're looking to work on your pressure flaking, you'll like the way this obsidian flakes. Obsidian makes beautiful arrow heads, why not try some today?