Geology Home Page physical geology historical geology
Roger Weller, geology instructor regional geology planetary gems
by Marciela Cordero
Obsidian is a volcanic glass that is formed when viscous (fluid) lava is
cooled so rapidly that it does not have a chance to crystallize into a specific
magnesium give the obsidian a dark green to black color.
There are several varieties of
Obsidian. Snowflake Obsidian,
Rainbow Obsidian, Sheen Obsidian, Midnight Lace Obsidian, Apache Tears, and
Mahogany Obsidian are a few of the many varieties of obsidian. Obsidian can
contain small bubbles of air that are aligned along layers created as the molten
rock flows before being cooled. These bubbles can produce interesting effects
such as a golden sheen, known as Sheen Obsidian or a rainbow sheen called
Inclusions of small, white, radially
clustered crystals of
in the black glass produce a blotchy or snowflake pattern producing Snowflake
snowflakes in this variety of Obsidian are Feldspar Crystals.
are small nuggets of obsidian
that have been naturally rounded and smoothed by wind and water. The legend
behind the Apache Tears relates that “About 75 Apaches and the US Calvary
squared off against each other in battle on a mountain overlooking what is now
Superior, Arizona in the 1870's. Rather than face defeat, the outnumbered Apache
warriors rode their horses off the mountain to their deaths. The families of the
warriors cried when they learned of the tragedy. Their tears turned into stone
upon hitting the ground. Today these beautiful translucent gemstones are known
as Apache Tears Good Luck Stones.” These stones are often given as gifts or used
as good luck charms.
Mahogany Obsidian often gets its
coloration from oxidized iron. Its name comes from the similar coloration it
shares with mahogany wood. In the case of obsidian, the slow flow of stiff,
viscous magma away from the source vent provides the mixing needed to create the
layered or streaked varieties of obsidian.
The "midnight lace" variety of
obsidian often has incredibly contorted streaking, apparently formed as the
obsidian layers are stretched and rolled with slow movement of the magma.
The various colors of
obsidian are a result of several factors. Clear varieties of obsidian contain
very few opaque impurities or microscopic mineral crystals. Red or brown
obsidian generally results from tiny crystals or inclusions of hematite or
limonite (iron oxide). Abundant, microscopic crystals of minerals like
magnetite, hornblende, pyroxene, plagioclase and biotite, combined with tiny
fragments of rock, likely produce the jet-black varieties of obsidian.
Microscopic crystals of various types of feldspars may yield the unique blue,
green, purple or bronze colors associated with rainbow obsidian. The reflectance
of rainbow obsidian is likely attributed to a preferred orientation of the
microscope crystals of feldspar or mica oriented along flow layers.
Obsidian has many
different uses. It is often made into art, jewelry, statues, and decorations.
Often confused with
obsidian has similar properties to
of a similar chemistry. However, many properties dependant on a crystal
structure are altered or absent in obsidian because it lacks any crystal
structure of its own. The piezoelectric and optical properties in quartz are
thus absent in obsidian. Smoky quartz usually has a splotchy or zoned
distribution to its color while Obsidian's color is more uniformly distributed.
Like All Glass and some
other types of naturally occurring rocks, obsidian breaks with a characteristic
“conchoidal” fracture. This smooth, curved type of fracture surface occurs
because of the near-absence of mineral crystals in the glass. The intersections
of conchoidal fracture surfaces can be sharper than a razor. This had obvious
advantages for our Stone Age ancestors, who used obsidian extensively for tool
making. Obsidian has been used by ancient people as a cutting tool, for weapons,
and for ceremonial purposes and is sometimes found by archaeologists in
archaeologists working at the site of Tlapacoya, southeast of Mexico City,
uncovered a well-made blade of obsidian associated with a radiocarbon date of
about 21,000 BC. Sharp shards of obsidian were formed into arrowheads by
Indians, who obtained large quantities of obsidian from Obsidian Cliff in
Yellowstone National park. The Aztecs used a great deal of obsidian for tools,
including sacrificial knives, the eyes of carved of their gods, and mirrors.
Obsidian occurs only where geologic
processes create volcanoes and where the chemical composition of the magma is
rich in silica. Obsidian-bearing volcanoes are typically located in or near
areas of crustal instability or mountain building. In North America, obsidian is
found only in localized areas of the West, where the processes of plate
tectonics have created geologic conditions favorable to volcanism and the
formation of obsidian. Obsidian typically forms near the end of a volcanic cycle
and is often associated with domes of volcanic cycle and is often associated
with domes of volcanic rock, such as the hills of Glass Buttes, Oregon.