Obsidian
:''This article is about volcanic glass. For other uses see
obsidian (disambiguation).
 |
Obsidian from Lake County, Oregon |
Obsidian is a type of naturally occurring
glass, produced by
volcanoes (
igneous origin) when a
felsic lava cools rapidly and freezes without sufficient time for crystal growth (see
glass transition temperature). It is commonly found within the margins of felsic lava flows, where cooling is more rapid.
While a rock like
basalt is dark because of
ferromagnesian enrichment, despite its dark color, obsidian consists mainly of SiO
2 (
silicon dioxide), 70% or more. Obsidian is
mineral-like, but not a true mineral because it is not
crystalline. Its composition is very similar to that of
granite and
rhyolite. It is sometimes classified as a
mineraloid. Because obsidian is
metastable at the earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than
Cretaceous age. The breakdown of obsidian is accelerated by the presence of water.
Tektites were once thought by many to be obsidian produced by
lunar volcanic eruptions, though few scientists now adhere to this hypothesis.
While pure obsidian is always dark in appearance, the color varies depending on the presence of impurities.
Iron and
magnesium typically give the obsidian a dark green to brown to black color. In some stones, 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). Obsidian is relatively soft with a typical
hardness of 5 to 5.5. Its
relative density is approximately 2.6.
Obsidian was highly valued in certain
Stone Age cultures because, like
flint, it can be fractured to produce sharp blades or
arrowheads. 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.
 |
Pig carved in snowflake obsidian, 10 cm (4 inches) long. |
Pre-Columbian Mesoamericans' use of obsidian was extensive and sophisticated, including carved and worked obsidian for
tools and decorative objects (see
Obsidian use in Mesoamerica). The ancient Mesoamericans also made a type of
sword with obsidian blades mounted in a wooden body. Called a
Maquahuitl, the weapon was capable of inflicting terrible injuries, combining the sharp cutting edge of an obsidian blade with the ragged cut of a serrated weapon. Several eyewitness reports claim to witness it cleaving off opponents' limbs, heads, and even the head of an unlucky cavalry horse.
Obsidian is used in
cardiac surgery, as well-crafted obsidian blades have a cutting edge up to five times sharper than high-quality steel surgical
scalpels, with the edge of the blade reaching veritable molecular thinness. It produces a cleaner cut and less tissue
trauma, which translates to faster healing and less
scar tissue.
Obsidian is also used for ornamental purposes and as a
gemstone, 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 a beautiful jet black; when cut across another direction it is glistening gray. "
Apache tears" are small rounded obsidian nuggets embedded within a greyish-white
perlite matrix.
*
Obsidian use in Mesoamerica*
Hyaloclastite,
tachylite - volcanic glasses with
basaltic composition
*
Flint knapping*
USGS Obsidian photo glossary*
Jim Miller, Obsidian is Hot Stuff, Volcano World*
Obsidian: Mineral galleries
