Stone of the Week - Obsidian

Obsidian forms from the melting of granitic rocks deep below the earth’s surface. It cools so quickly that it cannot crystallize, and instead forms as a volcanic glass. Since it has no crystalline structure, it is considered a mineraloid rather than a true mineral. [A mineraloid is something that is mineral-like but has no crystalline structure, such as amber, opal, and obsidian]. Obsidian is also known as volcanic glass, Apache Tears, Bergmahogany, and Black Lava Glass.

Obsidian comes in a variety of colors including black, brown, reddish brown, and dark green.

While the most common obsidian is black in color, it can also be found in brown, dark green, and reddish brown varieties (sometimes called Mahogany Obsidian). Another variety, and the most valuable, is called rainbow obsidian. Rainbow obsidian contains gas bubbles which formed layers or bands in the lava; when cooled the bands of bubbles remained and can reflect light forming a sheen of rainbow colors. [Fensterdog on Etsy asked about a golden or silver sheen - this is also due to the gas bubbles reflecting light.] Sometimes the hot lava can begin to cool and the formation of crystals begins, but is cooled quickly enough they do not continue to grow, resulting in snowflake obsidian. The little white/gray patches are small crystals of a mineral called cristobalite.

Obsidian can also form two special types called Rainbow Obsidian (left), and Snowflake Obsidian (right).


Obsidian has a conchoidal fracture, which usually looks smooth and spherical, and can create very sharp edges – so sharp that obsidian has been used for hundreds of years as cutting implements. Many indigenous tribes used it for arrow points and knives, as well as for decorative uses. It was a valuable commodity and traded between Native Americans, as well as tribes on other continents. Obsidian has unique properties that can be associated with particular volcanoes, which has made it possible for archaeologist to trace where some artifacts originated from. Today obsidian is used for surgical cutting implements, and more commonly as a gemstone for jewelry.

Obsidian has been used as arrowhead points by tribes in the past, and today is commonly used in jewelry.



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Obsidian Facts:
Chemical composition: Mostly SiO2, with MgO, Fe3O4
Crystal System: na
Color: black, brown, dark green, reddish brown
Habit: massive
Fracture: Conchoidal
Cleavage: na
Luster: Vitreous
Hardness: 5-5.5
Specific Gravity: ~2.6
Streak: white
Occurrence: Obsidian can be found around the world where lava has erupted at the surface and cooled quickly. Outcrops can be found in the western US, Armenia, Turkey, Italy, Mexico, Greece, and Scotland.



References
A Guide to Rocks and Fossils by B. Busbey III, R. R. Coenraads, P. Willis, and D. Roots. Published 2002 by Fog City Press. ISBN: 1877019518

Mindat – Obsidian. http://www.mindat.org/min-8519.html

Wikipedia – Obsidian. http://en.wikipedia.org/wiki/Obsidian