Obsidian

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For other uses, see Obsidian
(disambiguation).

Obsidian

Obsidian from Lake County,
Oregon

General

Category

Volcanic glass

Chemical
formula

70–75% SiO2,
plus MgO, Fe3O4

Identification

Color

Black

Fracture

Conchoidal

Mohs
scale hardness

~ 5 to 5.5

Luster

Vitreous

Specific
gravity

~ 2.5

Optical properties

Translucent

Obsidian is a naturally occurring volcanic glass formed as
an extrusive igneous
rock.

It is produced when felsic lava extruded from a volcano cools
rapidly with minimum crystal growth. Obsidian is commonly found within the
margins of rhyolitic lava flows known as
obsidian flows, where the chemical composition (high silica content) induces
a high viscosity and polymerization degree of the lava. The inhibition of atomic
diffusion through this highly viscous and polymerized lava explains the lack of
crystal growth. Obsidian is hard and brittle; it therefore fractures with very
sharp edges, which had been used in the past in cutting and piercing tools, and
are still used as surgical scalpel blades.[1]

Contents

1 Origin and
properties

2 Occurrence

3 Historical use

3.1 Middle
East

3.2 Americas

3.3 Easter
Island

4 Current use

5 See also

6 References

7 External
links

[edit]
Origin and
properties

Pliny's Natural
History features volcanic glass called "Obsidianus", so named
from its resemblance to a stone found in Ethiopia by one Obsius.[2]

Obsidian is mineral-like, but not a true
mineral because as a glass it is not crystalline; in addition, its
composition is too complex to comprise a single mineral. It is sometimes
classified as a mineraloid. Though obsidian
is usually dark in color similar to mafic rocks such as basalt, obsidian's
composition is extremely felsic. Obsidian consists mainly
of SiO2 (silicon dioxide),
usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite.
Because obsidian is metastable at the Earth's
surface (over time the glass becomes fine-grained mineral crystals), no obsidian
has been found that is older than Cretaceous age. This
breakdown of obsidian is accelerated by the presence of water. Obsidian has low
water content when fresh, typically less than 1% water by weight,[3]
but becomes progressively hydrated when exposed to groundwater, forming perlite. Tektites were once
thought by many to be obsidian produced by lunar volcanic eruptions, though
few scientists now adhere to this hypothesis.

Pure obsidian is usually dark in appearance, though the color varies
depending on the presence of impurities. Iron and magnesium typically give the
obsidian a dark green to brown to black color. Very few samples are nearly
colorless. In some stones, the inclusion of small,
white, radially clustered crystals of cristobalite in the black
glass produce a blotchy or snowflake pattern (snowflake obsidian). It may
contain patterns of gas bubbles remaining from the lava flow, aligned along
layers created as the molten rock was flowing before being cooled. These bubbles
can produce interesting effects such as a golden sheen (sheen obsidian)
or an iridescent, rainbow-like
sheen (rainbow obsidian).

[edit]
Occurrence

Obsidian can be found in locations which have experienced rhyolitic
eruptions. It can be found in Argentina, Armenia, Azerbaijan, Canada, Chile, Greece, El Salvador, Guatemala, Iceland, Italy, Japan, Kenya, Mexico, New Zealand, Peru, Scotland and the United
States. Obsidian flows which may be hiked on are found within the
calderas of Newberry Volcano and Medicine Lake
Volcano in the Cascade Range of western
North America, and at Inyo Craters east of the
Sierra Nevada in California. Yellowstone
National Park has a mountainside containing obsidian located between Mammoth Hot Springs
and the Norris Geyser Basin,
and deposits can be found in many other western U.S. states including Arizona, Colorado, New
Mexico, Texas,
Utah, Washington,[4]
Oregon[5]
and Idaho. Obsidian
can also be found in the eastern U.S. state of Virginia.

Obsidian arrowhead

[edit]
Historical use

Obsidian was valued in Stone Age cultures because,
like flint, it
could be fractured to produce sharp blades or arrowheads. Like all glass and
some other types of naturally occurring rocks, obsidian breaks with a
characteristic conchoidal fracture.
It was also polished to create early mirrors.

Modern archaeologists have
developed a relative dating system,
obsidian hydration
dating, to calculate the age of obsidian artifacts.

[edit]
Middle East

In Ubaid in the 5th millennium BC,
blades were manufactured from obsidian mined in what is now Turkey.[6]
Ancient
Egyptians used obsidian imported from the eastern Mediterranean and southern
Red Sea
regions. Obsidian was also used in ritual circumcisions because of its deftness
and sharpness.[7]

Obsidian talus at Obsidian Dome,
California

[edit]
Americas

Obsidian worked into plates and other wares by
Victor Lopez Pelcastre of Nopalillo, Epazoyucan, Hidalgo. On display at the Museo de
Arte Popular, Mexico City.

Raw obsidian and obsidian blades from the Mayan
site of Takalik Abaj

Lithic analysis can be instrumental in understanding prehispanic groups in
Mesoamerica. A careful analysis of obsidian in a culture or place can be of
considerable use to reconstruct commerce, production, distribution and thereby
understand economic, social and political aspects of a civilization. This is the
case in Yaxchilán, a Maya city
where even warfare implications have been studied linked with obsidian use and
its debris.[8]
Another example is the archeological recovery at
coastal Chumash sites in California
indicating considerable trade with the distant site of Casa Diablo,
California in the Sierra Nevada
Mountains.[9]

Pre-Columbian Mesoamericans' use of
obsidian was extensive and sophisticated; including carved and worked
obsidian for tools
and decorative objects. Mesoamericans also made a type of sword with obsidian blades mounted
in a wooden body. Called a macuahuitl, the weapon
was capable of inflicting terrible injuries, combining the sharp cutting edge of
an obsidian blade with the ragged cut of a serrated weapon.

Native American people traded obsidian throughout the Americas. Each volcano and in
some cases each volcanic eruption produces a distinguishable type of obsidian,
making it possible for archaeologists to trace the origins of a particular
artifact. Similar tracing techniques have allowed obsidian to be identified in
Greece also as coming from Melos, Nisyros or Yiali, islands in the
Aegean
Sea. Obsidian cores and blades were traded great distances inland from the
coast.[citation
needed]

Glass Mountain, a large obsidian flow at Medicine Lake
Volcano

In Chile
obsidian tools from Chaitén Volcano
have been found as far away as in Chan-Chan 400 km north of
the volcano and also in sites 400 km south of it.[10][11]

[edit]
Easter Island

Obsidian was also used on Rapa Nui (Easter Island) for edged
tools such as Mataia and the pupils of the eyes of their Moai (statues).

[edit]
Current use

Obsidian has been used for blades in surgery, as well-crafted
obsidian blades have a cutting edge many times sharper than high-quality steel
surgical scalpels, the cutting edge of
the blade being only about 3 nanometers thick.[12]
Even the sharpest metal knife has a jagged, irregular blade when viewed under a
strong enough microscope; when examined
even under an electron microscope
an obsidian blade is still smooth and even. One study found that obsidian
incisions produced narrower scars, fewer inflammatory cells, and less granulation tissue in
a group of rats.[13]
Don
Crabtree produced obsidian blades for surgery and other purposes[12],
and has written articles on the subject.

Pig carved in snowflake obsidian, 10 centimeters (4
in) long. The markings are spherulites.

Obsidian is also used for ornamental purposes and as a gemstone. It possesses the
property of presenting a different appearance according to the manner in which
it is cut: when cut in one direction it is jet black; in another it is
glistening gray. "Apache tears" are small
rounded obsidian nuggets embedded within a grayish-white perlite matrix.

Plinths for audio
turntables have been made of obsidian since the 1970s; e.g. the
greyish-black SH-10B3 plinth by Technics.

[edit]
See also

Hyaloclastite and tachylite –
volcanic glasses with basaltic composition

Knapping

Obsidian
hydration dating

Yaxchilan Lintel
24 – Ancient carving showing a Maya bloodletting ritual involving a rope
with obsidian shards.

[edit]
References

Notes

^ Brian Cotterell; Johan Kamminga (1992). Mechanics of pre-industrial technology: an introduction to the
mechanics of ancient and traditional material culture. Cambridge
University Press. pp. 127–. ISBN 978-0-521-42871-2. http://books.google.com/books?id=0-xOM8y6Uc8C&pg=PA127. Retrieved 9 September 2011.

^ Encyclopedia.com

^ "Perlite – Mineral Deposit Profiles, B.C. Geological
Survey". http://www.em.gov.bc.ca/mining/GeolSurv/MetallicMinerals/MineralDepositProfiles/profiles/r12.htm. Retrieved 2007-11-20.

^ Washington Obsidian Source Map

^ Oregon Obsidian Sources

^ "In Syria, a Prologue for Cities". The New York Times.
5 April 2010. http://www.nytimes.com/2010/04/06/science/06archeo.html

^ Harrell, James A. (2000). "Stone in Ancient Egypt". University of Toledo. http://www.eeescience.utoledo.edu/faculty/harrell/egypt/Stone%20Use/Harrell_Stones_text.htm.

^ Brokmann,
Carlos, Tipología y análisis de la obsidiana de Yaxchilán, Chiapas,
Colección Científica, no.422, INAH, 2000, 284p.

^ C.Michael Hogan (2008) Morro Creek, ed. by A.
Burnham

^ (Spanish) Pino, Mario and
Navarro, Rayen. Geoarqueología del sitio arcaico Chan-Chan 18. Revista
Geológica de Chile, 2005.

^ Naranjo, José A; Stern, Charles R (December 2004). "Holocene tephrochronology of the southernmost part (42°30'-45°S)
of the Andean Southern Volcanic Zone". Revista geológica de Chile
(Revista geológica de Chile) 31 (2): 225–240. doi:10.4067/S0716-02082004000200003. ISSN 0716-0208. OCLC 61022562. http://www.scielo.cl/scielo.php?pid=S0716-02082004000200003&script=sci_arttext. Retrieved 2008-05-02.

^ a
b
Buck, BA (March 1982). "Ancient Technology in Contemporary Surgery". The Western
journal of medicine 136 (3): 265–269. ISSN 0093-0415. OCLC 115633208. PMC 1273673. PMID 7046256. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1273673.

^
Disa, JJ; Vossoughi, J; Goldberg, NH (October
1993). "A comparison of obsidian and surgical steel scalpel wound
healing in rats". Plastic and reconstructive surgery 92 (5):
884–887. doi:10.1097/00006534-199392050-00015. ISSN 0032-1052. OCLC 121212765. PMID 8415970. http://www.ncbi.nlm.nih.gov/pubmed/8415970?dopt=AbstractPlus. Retrieved 2007-11-20.

[edit]
External links

Wikimedia
Commons has media related to:

Obsidian
(category)

Mindat with location data

Obsidian: Mineral galleries

USGS Obsidian photo glossary

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