Obsidian
Usually dark, the color varies on the presence of different materials. Iron and magnesium typically give the obsidian a dark green to brown to black color. A few samples are nearly colorless. In some stones, small crystals produce a snowflake pattern (snowflake obsidian). It may contain patterns of gas which produce effects such as a golden sheen (sheen obsidian) or a rainbow sheen (rainbow obsidian).