Obsidian is a volcanic glass that is found in the western USA and Mexico. It's well known for its ease of work ability thus making it a great choice for any skill level knapper. Your purchase will contain 10lbs of premium (Grade-A) spalls that range from 4-7 inches long and are a great choice for the beginner to expert knapper. Stone shown in the picture is an example of what to expect. Actual pieces may very. Skill Range: Beginner to Expert.