Obsidian

Obsidian is volcanic glass: Lava that has cooled very quickly, minimizing crystal formation. Quartz is all crystals.
Each volcano has a distinct composition of lava, and therefore obsidian. When we find obsidian artifacts in archaeological sites here on Vancouver Island, we know it didn't come from here as 'we got no volcanoes'. We can send the piece to a lab and have it tested, then trace it to it's home volcano. Helps us to figure out ancient trade routes, etc. Obsidian was highly prized as it produced extremely sharp cutting tools.