Writing on the Treehouse Wall


Obsidian is an igneous rock that is formed when felsic lava from a volcano is cooled so rapidly that the atoms are not able to form into a crystalline structure. Obsidian is mineral-like, but is not considered a true mineral because a glass cannot be crystalline in structure and it is too complex to be a single mineral. Obsidian is usually an extrusive rock – one that forms above Earth’s surface. Some of the places obsidian is known to form include along the edges of lava flows, where lava contacts water, and where lava cools while airborne.

Due to its unique composition obsidian is chemically unstable. As time passes obsidian begins to crystallize, a process that is accelerated by water. But crystallization doesn’t occur at a predictable and uniform rate throughout the rock. Obsidian crystallizes at random places within the rock creating radial clusters of grey or white cristobalite crystals. When cut and polished these specimens are known as “snowflake obsidian”. Eventually the obsidian will become fine-grained mineral crystals. Because of this process no obsidian has ever been found that is older than the Cretaceous period.

Snowflake obsidian is not the only interesting form of the rock. It is commonly known to form in colors such as black, brown and green. And on rare occasions it can form into blue, red, orange, or yellow obsidian. Sometimes two different colored obsidians will swirl together into one specimen. Black and brown are most commonly found together. This is called “mahogany obsidian”. But some of the most beautiful forms of obsidian are the “rainbow obsidian” and the “golden obsidian”. These have an amazing iridescent or metallic sheen to them caused by light reflecting off of tiny inclusions of crystals, rock, or gas caught in the specimen when it was cooling.

As beautiful as obsidian is it also has a practical use. When broken, obsidian fractures into pieces with curved surfaces caused by a conchoidal fracture. This kind of fracture can result in an extremely sharp edge. So sharp, it is possible to have a cutting edge many times sharper than high quality steel surgical scalpels. In one study they found the obsidian blade to leave narrower scars and less inflammatory cells. But despite the studies done on obsidian uses in surgery it has yet to be approved by the FDA.

Due to its many uses as a cutting tool and its easy identification, obsidian was one of the first targets of organized “mining”. The use of obsidian tools dates back to the Stone Age. The people used obsidian for arrowheads, knife blades, spear points, and scrapers. The discovery of tons of obsidian flakes suggests the presence of ancient factories that were probably operational for decades. With obsidian having so many uses to the ancient people it was very valuable and traded often. Obsidian and obsidian objects have been found up to a thousand miles from its source.



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