What is Obsidian?
Obsidian is an igneous rock that forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture (see photo on right).
Where Does Obsidian Form?
Obsidian is usually an extrusive rock - one that solidifies above Earth's surface. However, it can form in a
variety of cooling environments:
- along the edges of a lava flow (extrusive)
- around the edges of a sill or a dike (intrusive)
- where lava contacs water (extrusive)
- where lava cools while airborne (extrusive)
What Color is Obsidian?
Black is the most common color of obsidian. However, it can also be brown or green. Rarely, obsidian can be blue,
red, orange or yellow. The colors are thought to be caused mainly by trace elements or inclusions.
Occasionally two colors of obsidian will be swirled together in a single specimen. The most
common color combination is black and brown obsidian swirled together - that's called "mahogany obsidian" (see photo).
As a "glass", obsidian is chemically unstable. With the passage of time, some obsidian begins to crystallize.
This process does not happen at a uniform rate throughout the rock. Instead it begins at various locations within the rock.
At these locations the crystallization process forms radial clusters
of white or gray cristobalite crystals within the obsidian. When cut and polished these specimens are referred to as
"snowflake obsidian" (see photos at right).
Rarely, obsidian has an iridescent or metallic "sheen" caused by light reflecting from minute inclusions of mineral crystals,
rock debris or gas. These colored specimens are known as "rainbow obsidian", "golden obsidian" or "silver obsidian", depending upon
the color of the sheen or iridescence. These specimens are very desirable for the manufacture of jewelry.
|A baroque cabochon of iridescent "rainbow obsidian".
What is the Composition of Obsidian?
Most obsidians have a composition similar to rhyolite and granite. Granites and rhyolites can form from the same magma as obsidian and are often geographically associated with the obsidian.
Rarely volcanic glasses are found with a composition similar to basalt and gabbro. These glassy rocks are named "tachylite".
Are There Other Glassy Igneous Rocks?
Pumice, scoria and tachylite are other volcanic glasses formed by rapid cooling. Pumice and scoria differ from obsidian by having abundant vesicles - cavities in the rock produced when gas bubbles were trapped in a solidifying melt. Tachylite differs in composition - it has a composition similar to basalt and gabbro.
Occurrence of Obsidian
Obsidian is found in many locations worldwide. It is confined to areas of geologically recent volcanic activity. Obsidian
older than a few million years is rare because the glassy rock is
rapidly destroyed or altered by weathering, heat or other processes.
Significant deposits of obsidian are found in Argentina, Canada, Chile, Ecuador, Greece, Guatemala, Hungary, Iceland,
Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, Peru, Russia, United States, and many other locations.
In the United
States it is not found east of the Mississippi River as there is no geologically recent volcanic activity there. In the
western US it is found at many locations in Arizona, California, Idaho, Nevada, New Mexico, Oregon, Washington, and Wyoming.
Most obsidian used in the jewelry trade is produced in the United States.
Uses of Obsidian as a Cutting Tool
The conchoidal fracture of obsidian causes it to break into pieces with curved surfaces. This type of fracturing can produce
rock fragments with very sharp edges. These sharp fragments may have prompted the first use of obsidian by people.
The first use of obsidian by people probably occurred when a sharp piece of obsdian was used as a cutting
tool. People then discovered how to skillfully break the obsidian to produce cutting tools in a variety of shapes.
Obsidian was used to make knives, arrow heads, spear points, scrapers and many other weapons and tools.
Once these discoveries were made, obsidian quickly became the raw material of preference for producing almost any sharp object.
The easy-to-recognize rock became one of the first targets of organized "mining". It is probably a safe bet that all natural
obsidian outcrops that are known today were discovered and utilized by ancient people.
Stone Age Manufacturing and Trade
The manufacture of obsidian tools by humans dates back to the Stone Age. At some locations, tons of obsidian flakes reveal
the presence of ancient "factories." Some of these sites have enough waste debris to suggest that many people labored there for
decades producing a variety of obsidian objects. Making arrowheads, spear points, knife blades and scrapers from obsidian, chert or flint might
have been the world's first "manufacturing industry".
Obsidian was so valued for these uses that ancient people mined, transported and traded obsidian and obsidian objects
over distances of up to a thousand miles. Archaeologists have been able to document the geography of this trade by
matching the characteristics of obsidian in outcrops with the characteristics of obsidian in cutting tools. A study done by
the Idaho National Laboratory used composition studies by X-ray
fluorescence to identify the source outcrops of obsidian artifacts and map their use across the western United States.
Obsidian in Modern Surgery
Although using a rock as a cutting tool might sound like "stone age equipment", obsidian continues to play an important
role in modern surgery. Obsidian can be used to produce a cutting edge that is thinner and sharper than the best
surgical steel. Today, thin blades of obsidian are placed in surgical scalpels used for some of the
most precise surgery. In controlled studies, the performance of obsidian blades was equal to or superior to the performance
of surgical steel (reference 1, 2).
Uses of Obsidian in Jewelry
Obsidian is a popular jewelry stone. It is often cut into beads and cabochons or used to manufacture tumbled stones.
Opaque obsidian is sometimes faceted and polished into highly reflective beads. Some transparent specimens are faceted to produce
The use of obsidian in jewelry can be limited by its durability. It has a hardness of about 5.5 which makes it easy to scratch.
It also lacks toughness and is easily broken or chipped upon impact. These durability concerns make obsidian an inappropriate
stone for rings and bracelets. It is best suited for use in low-impact pieces such as earrings, brooches and pendants.
Obsidian is also used in making opal doublets and opal triplets. Thin slices or chips of opal are glued to a thin
slice of obsidian to make a composite stone. The black obsidian provides an inexpensive and color-contrasting background that makes
opal's colorful fire much more obvious. It also adds mass and stability to the opal that facilitates cutting it into a gem.
|A thin piece of obsidian is often used as a "backing" material for opal doublets and triplets. The black obsidian adds stability to the opal and provides a dark background color that contrasts with the opal's fire.
Other Uses of Obsidian
Freshly broken pieces of obsidian have a very high luster. Ancient people noticed that they could see a reflection in obsidian
and used it as a mirror. Later, pieces of obsidian were ground flat and highly polished to improve their reflective abilities.
Obsidian's hardness of 5.5 makes it relatively easy to carve. Artists have used obsidian to make masks, small sculptures and figurines for thousands of years.
Contributor: Hobart King
Find it on Geology.com
More from Geology.com
|Mount Etna: The most active volcano in Europe, continues an eruption that started in 2001.
|Plate Tectonics: Articles and teacher resources for plate tectonics and the interior of Earth.
|Obsidian: The specimen shown above is about two inches (five centimeters) across. The curved semi-concentric ridges are breakage marks associated with obsidian's conchoidal fracture. The rock has very sharp edges.
|Obsidian along the edge of a lava flow in central Oregon. Image © Phil Augustavo, iStockphoto.
|A tumble-polished specimen of "mahogany obsidian". Image © Arpad Benedek, iStock.
|A tumble-polished specimen of "snowflake obsidian". Image © Martin Novak, iStockphoto.
|A knife blade manufactured from mahogany obsidian. The craftsman who made this blade had a very high skill level and was able to produce a serrated edge. Image © Al Braunworth, iStockphoto.
|A spear point fashioned from opaque black obsidian. Image © Charles Butzin, iStockphoto.
|Mahogany obsidian and snowflake obsidian cabochons set in a sterling silver pendants.