Obsidian: a glass, not a mineral, and why that matters

Obsidian is volcanic glass, formed when lava cools so quickly that no crystal structure has time to form, which technically disqualifies it from being a true mineral even though it is collected and cut alongside gemstones constantly. It breaks with an extremely sharp conchoidal fracture, sharp enough that obsidian blades have been used as cutting tools throughout human history and are still used in some specialty surgical blades today because the edge can be thinner than steel. Apache tears are a well known form, small rounded nodules of obsidian found embedded in a lighter colored volcanic ash matrix called perlite, and they are translucent at thin edges despite looking solid black in hand. Because it is glass, obsidian has no definite hardness scratch pattern the way crystalline minerals do, and it will not show any crystal faces no matter how a piece breaks.