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Roger Weller, geology instructor regional geology planetary gems

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Obsidian
by Marciela Cordero
Physical Geology
Spring 2008
Obsidian

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 mineral. Iron and 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 Rainbow Obsidian.

Inclusions of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern producing Snowflake Obsidian. The snowflakes in this variety of Obsidian are Feldspar Crystals.

Apache Tears 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 smoky quartz, obsidian has similar properties to quartz because 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 excavations.

In 1967 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.