OBSIDIAN

-Composition: Silica (volcanic glass)
 -Hardness: 6-7
-Occurence: Obsidian occurs in volcanic lava flows, from silica-rich magma that
  has come to the surface.
-Colour: black,mahogany, snowflack, greenish
-Cleavage: Obsidian has a conchoidal fracture. Which means it has curved sides.
-Formed: The lava cools
 

For more information check out this link: www.rockgallery.com