Obsidian is often the result of volcanic lava coming in contact with water (a lake or an ocean), and thus being cooled too quickly to form a clear crystalline structure. The process does produce a glassy texture in the resulting rock.
Obsidian said to be a very powerful stone, reminding one that birth and death are simultaneous and constantly present, not as one with the other, but always as one. Obsidian has been associated with guardian spirits for as far back as ancient lore provides. Said to watch over one, protect, and connect to protection on all levels of experience.