Obsidian

Obsidian is a volcanic glass from magma chambers with a lot of silica in them. The silica being lighter than other minerals in the lava floats to the top and when the eruption happens just right you get glassy obsidian eruptions. 

Obsidian can also form as apache tears in certain silica rich tuffs as little glass balls. 

Obsidian comes in black, pumpkin orange, various sheens, fire and an olive to light olive drab green. 

Sheen obsidians get their color from various size bubbles in the layers reflecting colors back. Sheen can be any color in the rainbow or metallic gold/silver. 

Fire obsidian gets its color from magnetite layers in the obsidian and flashes back a metallic vibrant colorful reflection of all colors and crazy patterns