Obsidian

Top stone is obsidian, below that it is pumice and in lower right hand is rhyolite (light color)Obsidian is one type of naturally occurring glass, which produced from volcanoes (igneous rock) when a fluid felsic lava cools rapidly and then freezes without sufficient time for crystal growth, for example by flowing into a body of the water. It consists mainly of SiO2 (silicon dioxide), 80% or more. Obsidian is a mineral-like, but not a true mineral because it is not a crystalline. Its composition is very similar to that of granite and also rhyolite. It is sometimes also classified as a mineral aid.

Colour and Value

The color of obsidian varies depending on the presence of its impurities. Iron and magnesium typically give the obsidian a very dark green to brown to black color. The inclusion of small, white, radically clustered crystals of the cristobalite in the black glass produce in a blotchy and snowflake pattern (snowflake obsidian). It can also contain patterns of gas bubbles remaining from the lava flow, which aligned along layers created as the molten rock was flowing before being cooled. These bubbles could produce interesting effects such as a golden (sheen obsidian) and rainbow sheen (rainbow obsidian). Small nuggets of obsidian that have been naturally rounded and then smoothed by wind and water are called "Apache tears." Obsidian is normally relatively soft with a typical hardness of 5 to 6. Its specific gravity could be approximately 2.6.

Obsidian is commonly used for making ornamental and as a gemstone, for it possesses the peculiar property of presenting in a different appearance according to the manner in which it is cut. When cut in one direction it is a very beautiful jet black; when cut across another direction it is glistening gray.

Obsidian was highly valued during Stone Age cultures because, like flint, it could be fractured to produce sharp blades and arrowheads. Like all glass and some other types of naturally occurring rocks, obsidian breaks with the characteristic concordat fracture. It can also have been polished to create early mirrors.

Obsidian is currently used in the cardiac surgery, as well-crafted obsidian blades have a cutting edge much sharper than the high-quality steel surgical scalpels, and can even be up to five times sharper, the edge of the blade reaching veritable molecular thinness. It produces a cleaner cut and very less tissue trauma, which translates to faster healing and less scar tissue.