Obsidian - 100 pounds*Natural Obsidian from Mexico 100 pounds* Mostly large pieces *Requires 150lbs of shipping space Obsidian is a naturally occurring volcanic glass that typically forms when lava cools rapidly without sufficient time for crystal growth. Color: Obsidian comes in various colors, including black, brown, gray, and green. The most common is black obsidian, which is often referred to simply as "obsidian." Hardness: Obsidian has a hardness of approximately 5 to