Obsidian is actually a naturally forming glass. When viscous lava cools rapidly, it creates this type of igneous rock. It cools so fast that crystals are unable to form, meaning that obsidian is brittle and amorphous. This feature means that when obsidian is cleaved, it breaks off into very sharp edges, hence why this volcanic glass has been used as weapons and knives for centuries (and is, in fact, still used today in surgical tools!).
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I like working wirh rainbow and metallic sheen varieties. Rainbow obsidian occurs when various layers of lava cool at different rates. These layers each have their own concentration of magnetite nanoparticles which interplay with light differently. Therefore, you can see various bands inside rainbow obsidian, and some show up as different colors (and it is EXCEEDINGLY hard to photograph). Although you can find obsidian in almost every country with volcanoes, rainbow obsidian is almost exclusively mined in California.
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Metallic sheen (golden, silver, and copper) obsidian is slightly different in that its flash of color comes from tiny bubbles that were formed as the volcanic glass quickly cooled. Since obsidian has magnetite nanoparticles, these gaseous inclusions react with light as a metallic sheen.