my husband is a geologist, i bet he would love to answer this for you!! sorry, but just had to let you know if he was here, i'd have him answer it, do you want me to ask him later?

Here he is without further ado, my husband, Dr. Rocks: Obsidian is a Volcanic Igneous rock, thus it forms from the cooling of melted rock called Magma. Obsidian is a special type of volcanic rock based on composition and how it cools. It is composed of magma that contains over 75% silica dioxide (Glass). This makes the magma very thick and gooey. This type of magma is called Silicic. typically magmas of this type create the largest and most violent eruptions you could imagine; however, for an obsidian the eruption is fairly mild because the magma has very little disolved GAS (water mostly). so, the magma comes to the surface as a thick gooey mess and pushes out of the Earth like toothpaste. For the magma to cool and form a true obsidian, it must cool fairly quickly so that no silicate minerals can form. If it cools fast and no silicate minerals form, then the magma cools to a true GLASS! Remember, it is composed of over 75% glass. So, you end up with Obsidian that behaves as a true class with concoidal fracturing, etc. Native Americans used the property obsidian to make arrow heads (projectile points.). The best obsidian flows in the World are in California (Mono Lakes - Inyo Craters, and at Lava Beds National Monument in the NE corner of the state). Hope this helps, or you are sleeping now.

Isn't it obsidious?