Obsidian -- even the name is exotic. Ever since I had my first rock collection as a kid, I've loved obsidian. Sharp and shiny, obsidian is so different from other rocks. But until a few years ago when I made my first obsidian collecting trip to Glass Buttes, Oregon, I thought obsidian was pretty much just black glass. That trip really opened my eyes and inspired me with curiosity. The ancient volcanic hills called Glass Buttes hold a dazzling variety of gem-quality obsidian: mahogany, red, flame, midnight lace, jet black, pumpkin, brown, rainbow, gold sheen, silver sheen, green, lizard skin, snowflake and more. Since that trip I've learned a little more about this incredible stone we call obsidian. My goal in this article is to increase your awareness of some of the more fascinating aspects of obsidian, which is so prized by many knappers.

Those of you who have ever had more than a passing interest in geology were probably quickly discouraged by all the technical jargon. Instead of calling the first college-level course "Geology 101," they should call it "Terminology 101." I'll try to boil down the technical nomenclature to everyday terms, as best I can.

All knappable materials such as obsidian share one characteristic--they break with a "conchoidal" fracture. This smooth, curved type of fracture surface allows for a predictable and controllable flake to be detached (fractured) from a larger piece of rock. The intersections of conchoidal fractures can be sharper than a razor. This obviously had significant advantages for our Stone Age ancestors for many millennia.

Obsidian is natural glass that was originally molten magma associated with a volcano. Like all glass, obsidian has a conchoidal fracture because of an almost total absence of sizable mineral crystals within the rock matrix. When I say "crystals," don't think of those beautiful pointed prisms of quartz found in geodes. All rocks consist of mixtures of various crystalline minerals. When crystallization occurs, the atoms that comprise a mineral become arranged in regular, geometric patterns that are unique to the specific mineral. Crystal faces form only where there is enough open space in the rock mass to allow the natural geometric forms of the crystals to develop as free faces. Granite is composed entirely of small, intergrown crystals of quartz, feldspar, mica and other minerals. These relatively large mineral crystals (easily visible to the naked eye) give granite a rough fracture surface. Unlike obsidian, granite can't be flaked successfully by knapping because its crystals prevent conchoidal fracturing of the rock.

Obsidian consists of about 70 percent or more non-crystallized silica (silicon dioxide) and is chemically similar to granite and rhyolite. Obsidian is somewhat softer (5 to 5.5 on the mineral hardness scale) than most other knappable materials because of its near absence of mineral crystals. In contrast, flint is composed mainly of microscopic crystals of quartz, which is crystallized silica. These very small quartz crystals of similar size give flint its conchoidal fracture with a hardness similar to quartz (6.5 to 7 on the hardness scale) and a durability greater than obsidian.

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 rock, such as the hills of Glass Buttes, Oregon.

So, if obsidian is similar in composition to granite and rhyolite, both of which were originally molten, why is obsidian glassy? The answer relates to the original water content and cooling rate of the magma. Granite cools very slowly far below the surface of the earth; this allows the formation of sizable mineral crystals within the slowly congealing mass of molten rock. Rhyolite typically cools more rapidly near the earth's surface and contains smaller mineral crystals than granite. When rhyolite magma approaches the earth's surface and the pressure of burial decreases, most of the water in the magma is lost as steam. The resulting silica-rich magma with little remaining water becomes very viscous (thick and pasty) obsidian magma. This molten rock is so viscous that sizable mineral crystals cannot grow before chilling of the magma "freezes" crystal development.

Some obsidian is erupted as lava flows at the ground surface. These surface flows are so viscous that they flow very slowly. One article I read indicated that "an ant could probably outrun an obsidian lava flow." An excellent example of a relatively recent obsidian flow can be found at Paulina Lake (part of the Newberry Volcano), approximately 30 miles southeast of Bend, Oregon. Portions of this obsidian flow are mixed with layers of pumice, a glassy, bubble-rich, lightweight rock that develops when water vapor (steam) escapes rapidly from the molten rock at the ground surface.

Sometimes obsidian of excellent quality develops as surface lava flows. However, the best quality obsidian often forms below the surface around a volcanic vent. 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 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, minute 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 microscopic crystals of feldspar or mica oriented along flow layers.

Changes in magma composition and water content often occur during the eruption and subsurface emplacement of obsidian flows. The high viscosity of the molten obsidian prevents effective mixing of these magmas, resulting in obsidian that is "streaked" with different layers or colors. Each of these streaks or lines may represent a distinct pulse of an obsidian eruption. You can visualize the process that results in streaked obsidian if you consider two blobs of green and red taffy (a viscous candy) that are mixed together. Distinct streaks of red and green taffy result as the blobs are mixed. 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 were stretched and rolled with slow movement of the magma.

A certain amount of water always is present in obsidian. Very small inclusions of water vapor in the form of bubbles often are trapped in the glass. Tiny gas bubbles that have been stretched nearly flat along the flow layers in obsidian generally cause the reflectance of gold sheen and silver sheen obsidian. Some of these bubbles are visible to the naked eye. The bubbles can be seen readily with a strong magnifying glass or a microscope.

Some obsidian alters to a glassy, gray-brown rock known as "perlite" through absorption of water during and after cooling. Water absorption starts along cooling fractures in the glass and proceeds as concentric circles expanding away from the fractures toward the solid cores of unfractured rock. When heated, perlite expands to many times its original volume as water vapor escapes the rock, forming an artificial, pumice-like material. The smooth pebbles of obsidian known as "Apache tears" represent the final remnants of obsidian inside the centers of these concentric circles of perlite. Lapidary articles indicate that some Apache tears have considerable internal pressures. Attempting to cut Apache tears with a rock saw often produces only flying chips of glass. I would suspect that knapping Apache tears would only hasten the accumulation of black glass shards on the floor.

Obsidian is relatively unstable from a geologic perspective. It is rare to find obsidian older than about 20 million years, which is very youthful in comparison to most continental rocks that form the Earth's crust. Over a long period of time, obsidian gradually changes from glass to rock in a process known as "devitrification." In this process, the silica molecules of glass slowly rearrange into organized crystal patterns. The multi-colored sheen on the surface of old glass bottles shows the early onset of devitrification. The "snowflakes" in snowflake obsidian are quartz crystals that have formed through devitrification of the original obsidian. The crystals that develop through devitrification cause obsidian to lose its conchoidal fracture and glassy texture.

Native Americans discovered almost all of the obsidian locations in North America. Each obsidian source area has a unique assemblage of trace elements, allowing identification of the original source locality for the obsidian used in prehistoric artifacts. Trace element analyses have shown that native peoples traded this valuable commodity many hundreds of miles from the volcanic source areas along numerous trade routes. The fact that obsidian was transported great distances attests to the mystique of this unique rock.

My favorite place to collect obsidian for knapping or lapidary work is Glass Buttes in central Oregon. A trip to Glass Buttes is well worth the effort for the natural scenery, abundant and beautiful gem-quality obsidian, and exploration of a fascinating geological area. Knappers have frequented these juniper and sagebrush covered hills for thousands of years. Located in central Oregon south of Highway 20 and about 80 miles east of Bend, a visitor will be rewarded with many different varieties of obsidian through exploration of the area. Fist-sized pieces are abundant and can be collected with no digging. For large pieces of high quality obsidian (ranging up to several hundred pounds), considerable effort with a shovel and pry bar are necessary. Further information on planning a trip to Glass Buttes can be found in CHIPS, Vol. 9, #2, 1997. The early part of Craig Ratzat's videotape titled Caught Knapping shows how to dig for and remove larger pieces of obsidian at Glass Buttes. I've been to Glass Buttes several times and likely will make an annual pilgrimage there. But don't worry, I'll leave some choice pieces for you.

* Jim Miller is a practicing geologist and an avid knapper residing in Bothell, Washington.

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