Obsidian is an igneous rock, a mix of cryptocrystalline grains of silica minerals in a glass-like suspension. It is a natural occurrence of volcanic glass created when lava from a volcano cools rapidly with minimal crystal growth. 

Gold Sheen Obsidian has an iridescent or metallic "sheen" caused by light reflecting from minute inclusions of mineral crystals, rock debris, or gas.

Green Obsidian is man-altered.
 
Mahogany Obsidian is a natural volcanic glass and can be found in areas with volcanic activity including Afghanistan, Japan, Mexico, South America, and parts of the western United States. It's a deep reddish-brown with black inclusions. The distinctive coloring comes from high concentrations of iron.
 
Snowflake Obsidian, also known as Flowering Obsidian and Spherulitic Obsidian, is natural volcanic glass containing white 'snowflake' crystal patterns of the mineral cristobalite, due to partial crystallization of the glass.
 
Since Obsidian is a natural glass it may have razor-sharp edges that can easily cut skin and flesh if broken. Handle with care. Do not grind dry since long-term exposure to finely ground powder may lead to silicosis.