glass, produced from volcanoes when the right kind of lava cools rapidly, e.g., by flowing into a body of water. It consists mainly of SiO2 (silicon dioxide), 70% or more. Obsidian is mineral-like, but not a true mineral because it is not crystalline. It is otherwise very similar in composition to granite and rhyolite It is sometimes classified instead as a mineraloid.
The color of obsidian varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark green to black color. The inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (Snowflake Obsidian). It may contain patterns of air bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden (sheen obsidian) or rainbow sheen (rainbow Obsidian). Small nuggets of obsidian that have been naturally rounded and smoothed by wind and water are called "apache tears." Obsidian is relatively soft with a typical hardness of 5 to 5.5. Specific gravity is approximately 2.6.
Obsidian is commonly used for ornamental purposes, for it possesses the peculiar property of presenting a different appearance according to the manner in which it is cut. When cut in one direction it is of a beautiful jetty black; when cut across another direction it is glistering gray.
Pig carved in snowflake obsidian,|
10 cm (4 inches) long.
Obsidian was highly valued in certain Stone Age cultures because, like flint, it can be fractured to produce sharp blades or arrow heads. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic conchoidal fracture. It may also have been polished to create early mirrors.
In Pre-Columbian Mesoamerican obsidian use was extensive and sophisticated with carved and worked obsidian for tools, as well as for decorative objects. Well crafted obsidian blades are capable of having a cutting edge as fine as high quality surgical steel scalpels. The ancient Mesoamericans also made a type of sword with obsidian blades mounted in a wooden body.