OBSIDIAN Rough * Choice of Sizes * Natural Black Igneous Knapping Volcanic Glass For Sale

This listing is for your choice of sizes of black Obsidian similar to the photos pictured from Modoc. County, California. These specimens are quite showy - especially with a fresh broken face (first photo) and would look great displayed on your mantle or bookcase. It is also popular knapping rough for making arrowheads, spear points, etc..
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OBSIDIAN PROPERTIES:
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. Obsidian is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) causes a high viscosity, which, upon rapid cooling, results in a natural glass forming from the lava. The inhibition of atomic diffusion through this highly viscous lava explains the lack of crystal growth. Obsidian is hard, brittle, and amorphous; it therefore fractures with sharp edges. In the past, it was used to manufacture cutting and piercing tools, and it has been used experimentally as surgical scalpel blades.

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