Obsidian has an Amorphous crystal system and is in the mineral class of Oxides. It is formed volcanically when lava that is rich in silicic acid cools very rapidly. It hardens so quick that no crystalline structures are able to form. The result is a solid, molten mass that contains large amounts of minerals, water and gas inclusions. It is sometimes also called volcanic glass.
As a rule, it is usually found as a very opaque, black color. When more transparent, it has been formed from lava that is poor in foreign substances. It can also be silvery in color (Snowflake) from fine gas bubbles included, or very iridescent in it’s color (Rainbow Obsidian) from trapped water bubbles.