Obsidian

Obsidian can be both extrusive and intrusive igneous rock because it can be formed from the sides of flowing lava and along the sides of sills and dikes, and when lava cools due to touching water or making contact with cooler air. All of these methods are extrusive except when lava cools along the sides of sills and dikes.

According to geology.com, "Significant deposits of obsidian are found in Argentina, Canada, Chile, Ecuador, Greece, Guatemala, Hungary, Iceland, Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, Peru, Russia, United States, and many other locations."

The most common characteristic of obsidian is its dark, glassy exterior but there is a rare occurrence of many other colors including green, red, blue, orange and yellow. Further, its composition resembles granite fairly often.

Obsidian is often used in jewelry but has been a vital material for many tools from early ages to the present day.

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