Igneous Rock: Obsidian

For my igneous rock, I chose obsidian because I have a few pieces at my house. You might know obsidian as "volcanic glass." Obsidian forms when lava cools at a rapid pace and prevents the formation of crystalline structures. It is generally an extrusive rock but can form as an intrusive rock in certain situations. It displays a conchoidal fracture when broken.

This is an image of lava cooling and forming obsidian.

Even though obsidian is black in most cases, it can also appear green or brown. In a low number of cases, obsidian can appear blue, red, yellow, and orange, and it can contain a mixture of colors. 

This particular piece of obsidian was shaped and smoothed into a jewelry stone and carries a mixture of color. In many pre-modern cultures, people used obsidian to create cutting tools and spearheads, as well as jewelry.  

Obsidian is abundant in areas of recent volcanic activity, such as Canada, Argentina, Peru, Russia, Greece, the United States, and several other locations. In the United States, obsidian is not present east of the Mississippi River due to the absence of volcanic activity. The picture of an obsidian deposit featured above was taken in Oregon.

These two images were also taken in Oregon, although in a different location. They display what is called the "Big Obsidian Flow" that resulted from an eruption of the Newberry Volcano in central Oregon around 700 C.E. 

If you would like to check these facts or learn more about obsidian, you can visit the following website:  http://geology.com/rocks/obsidian.shtml