Obsidian is natural glass that was originally molten magma associated with a volcano. This
volcanic glass has an almost total absence of sizable mineral crystals within the glass matrix.
When I say "crystals," don't visualize 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 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.
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 consists of about 70 percent or more non-crystallized silica (silicon dioxide). It is
chemically similar to granite and rhyolite, which also were originally molten. Because obsidian
is not comprised of mineral crystals, technically obsidian is not a true "rock." It is really a
congealed liquid with minor amounts of microscopic mineral crystals and rock impurities.
Obsidian is relatively soft with a typical hardness of 5 to 5.5 on the mineral hardness scale.
In comparison, quartz (crystallized silicon dioxide) has a hardness of 7.0.
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.
If obsidian is similar in composition to granite and rhyolite, both of which were
originally molten, then why is obsidian glassy? The answer relates to the original cooling rate
and water content of the magma. Granite cools very slowly miles below the surface of the earth;
this slow cooling over millions of years allows for the formation of sizable mineral crystals
within the slowly cooling 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 magma 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 glass at
or near the ground surface.
Sometimes obsidian of excellent quality develops as surface lava flows. However, the best
quality obsidian often forms below the ground surface around volcanic vents. Silica-rich magma
squeezes into rock fractures to form layers and lenses of obsidian that are relatively free of
dirt, ash and other impurities.
Several varieties of obsidian from Glass Buttes, Oregon and tools made by the author from those
materials. From left to right across the top, the obsidian varieties are
silver sheen, midnight lace, pumpkin and red mahogany.
Source material and stone tools of red mahogany obsidian.
Source material and stone tools of red mahogany obsidian. Pencil gives scale.
Source material and stone tool of silver sheen obsidian.
Source material and stone tools of pumpkin obsidian. Pencil gives scale.
Multi-colored streaks oriented along flow lines in midnight lace obsidian.
Contorted streaks within midnight lace obsidian.