OBSIDIAN

Description:

Obsidian is a type of naturally occurring glass, produced from volcanoes when a fluid lava cools rapidly and freezes without sufficient time for crystal growth, for example by flowing into a body of water. Obsidian consists of about 70 percent or more non-crystallized silica (silicon dioxide). It is chemically similar to granite and rhyolite, which also were originally molten. Because obsidian is not comprised of mineral crystals, technically obsidian is not a true "rock."

Obsidian occurs only where geologic processes create volcanoes and where the chemical composition of the magma is rich in silica. The best quality obsidian often forms below the ground surface around volcanic vents. Silica-rich magma squeezes into rock fractures to form layers and lenses of obsidian that are relatively free of dirt, ash and other impurities.

The color of obsidian varies depending on the presence of impurities. Iron and magnesium typically give a dark green to brown to black color. Red or brown obsidian generally results from tiny crystals or inclusions of hematite or limonite (iron oxide). Microscopic crystals of various types of feldspars may yield the unique blue, green, purple or bronze colors associated with rainbow obsidian.

There are several types of obsidian. 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 gas 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". The most notable occurrences of obsidian include Arizona, Colorado, Texas, Utah and Idaho, USA.

Obsidian was used by ancient people as a cutting tool, for weapons, and for ceremonial purposes. It was highly valued in certain cultures because it can be fractured to produce sharp blades or arrowheads. It may also have been polished to create early mirrors.

Obsidian is commonly used for ornamental purposes and as a gemstone due to its peculiar property of presenting a different appearance according to the manner in which it is cut: if cut in one direction it is a beautiful jet black; when cut across another direction it is glistening gray.

Moreover, obsidian is currently used in cardiac surgery, as well-crafted obsidian blades have a cutting edge up to five times sharper than high-quality steel surgical scalpels. It produces a cleaner cut and less tissue trauma, which translates to faster healing and less scar tissue.

Characteristics:

Chemistry: SiO₂; Mostly silicon dioxide with certain amounts of impurities

Class: Mineraloids

Hardness (Mohs scale): 5 – 6

Density: 2.3 – 3.0

Refractive index: 1.48 – 1.51

Dispersion: 0.010

Fracture: conchoidal

Transparency: translucent to opaque

Luster: vitreous