Obsidian

Obsidia is a type of naturally occurring 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. It is mineral-like, but not a true mineral because it's not crystalline. It is otherwise very similar in composition to granite and rhyolite It's sometimes classified instead as a mineraloid.

Pig carved in snowflake obsidian, 10 cm (4 inches) long. Larger version

Its colour varies depending on the presence of impurities. Iron and magnesium commonly 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 a 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.

It is a good deal 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 that direction it is glistering gray.

It 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 Mesoamerica made extensive and sophisticated use of carved 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.