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Obsidian is a natural stone formed by the rapid cooling of lava erupted from volcanoes; it has a glassy luster and is typically black in color. It appears beautiful and is also very hard, which is why humans have used it for both tools and jewelry. How Is Obsidian Formed? When volcanoes erupt, lava is expelled. If this lava cools very rapidly, the stone known as obsidian forms. Obsidian resembles glass because it contains no crystals, giving it a smooth and shiny surface. Colors of Obsidian Obsi