Obsidian is a volcanic stone formed by rapid cooling of lava on surface of the earth. Obsidian is also called natural glass and is a silicon dioxide.
Obsidian does not exhibit crystals unlike stones like quartz or amethyst. The reason is rapid cooling that does not allow for formation of crystals.
Obsidian is found in several varieties like black obsidian, snow flake obsidian and mahogany obsidian. Black obsidian is one of the most abundant varieties and was used as weapon and for hunting in prehistoric times. Arrow heads made of obsidian have been found in several parts of the globe that prove the importance of this stones to prehistoric humans.
The reason behind using obsidian as a weapon is the fact that when broken the edges are extremely sharp (just like manmade glass) and are ideal for making necessary tools.
Obsidian is one of the most affordable stones used in jewelry and also as an ornamental and decorative stone.
Obsidian is much more resistant to scratches than glass and can easily scratch it.
The luster of obsidian is very vitreous and its hardness on Mohs scale is something between 5.5 and 6.
Unpolished and broken obsidian should be handled with much caution and care because of the edges that are very very sharp and can cause serious injuries.