Obsidian

Chemical Composition: an Igneous Silica (volcanic glass)
Hardness: 6-7

Occurence: Obsidian occurs in volcanic lava flows, from silica-rich magma that has come to the surface.

Metaphysical Properties: Excellent for grounding and protection. Obsidian wards off negativity. Useful for introspection - helps one to see where changes need to be made within oneself.
Corresponding Astrological Sign: Sagittarius
Chakra: 1st - Root chakra