OBSIDIAN

( Fr- obsidienne/obsidiane; Ger- Obsidian/Feuerkiesel; Nor- obsidian; Rus )

OBSIDIAN



A. Mahogany obsidian (height - 7 cm), also called "Mountain mahogany," from the Jemez Mountains, New Mexico. (© photo by W. Sacco)

B. Snowflake obsidian (height - ca. 3.6 cm) from Black Rock, Milford, Utah. (© original photo [here irregularly cropped] by Frederick H. Pough)

C. Rainbow obsidian heart (width - 9 cm) from Mexico. Talisman Trading. (© photo by Jeffrey A. Scovil)

D. Obsidian hippopotamus (length - ca. 9 cm). Carving from brecciated obsidian by Gerd Dreher. Silverhorn. (© photo by Jeffrey A. Scovil)

E. Gold-sheen obsidian (height - 8 cm) from Mexico. Carving appears black when viewed from some directions. Rick Dietrich collection. (© photo by Dick Dietrich)

DESCRIPTION: Obsidian is natural glass formed by quenching (i.e., rapidly cooling) magma of granitic/rhyolitic or similar composition.
    Colors - commonly dark gray to nearly black, less commonly bluish gray or reddish brown and rarely with streaks of pastel hues that are pinkish, yellowish, greenish, purple, brown, etc.; some is iridescent, most commonly exhibiting silver or golden tones; some is mottled or roughly banded, the latter apparently representing flow patterns.
    H. ~ 5-5½
    S.G. 2.3-2.6
    Light transmission -
most is transparent to subtranslucent in thin slivers;  some reddish brown obsidian and a dark green glass, most of which is sideromelane rather than obsidian (see REMARKS) is virtually opaque
    Luster - vitreous or subvitreous
    Breakage - conchoidal fracture is typical
    Miscellany - some obsidian appears to be chatoyant, iridescent, or aventurescent because of the presence of minute inclusions of minerals -- e.g., hematite and/or ilmenite -- and/or  bubbles that are relatively abundant in some lamellae and less so or virtually lacking in intervening lamellae.   Streak -- i.e., powder --  of virtually all obsidians, even those that re nearly black, appears white.

OTHER NAMES: A few obsidian bodies have extents that have led to their being named according to the scheme used for stratigraphic units (see Appendix B, Glossary). Three examples are the East Lake Obsidian of Obsidian cliff, Yellowstone National Park, Wyoming; the Mono Craters Obsidian of the Sierra Nevada of California; and the Newberry Obsidian of Oregon.  Additional names, many of which have been given in the markeplace, follow:

USES: Obsidian is frequently carved for jewelry -- especially earrings, bracelets and pendants -- and into diverse ornaments, such as busts and animals, made especially for the souvenir trade, and rod and needlelike pieces of obsidian have been incorporated into wind chimes. Especially in ancient times, because it breaks to give sharp edges, obsidian was sought and used for cutting tools, weapons and ceremonial points such as spearheads;  indeed, even today replica arrow heads are made from obsidian for use as pendants or incorporation in decorative items, including jewelry.  In addition, obsidian has been carved into diverse vessels such as chalices and vases, and incorporated into statues (etc.) as pupils of eyes (Weiner, 1983), and Pough (ms) reports that "in Mexico, Eduardo Obe's, of Morellos creates giant sculptures for art galleries and tequila cups [from obsidian] for his opening night patrons."  

OCCURRENCES: As masses of rapidly cooled, silica-rich magma associated with relatively recent (geologically speaking) volcanism. MacDonald, Smith and Thomas (1992) list lava flows, extrusive domes and blocks in pyroclastic deposits as loci.

NOTEWORTHY LOCALITIES: Parts of Italy; the Caucasus Mountains of the former U.S.S.R; and areas in Mesoamerica -- e.g., the vicinity of Real Del Monte, State of Hidalgo, Mexico (especially for sheen obsidian) -- have been major sources.  Also, several localities in the western United States are well-known for one or another kind of obsidian that has been used as a gemrock -- e.g., Mariposa and Pinal counties, Arizona;  Glass Mountain, south of Lava Beds National Monument, Siskiyou County and near Davis Creek in adjacent Modoc County, northern California (especially for rainbow obsidian, but also for other varieties -- see Mitchell, 1987);  near Milford, Beaver County Utah (especially for rainbow obsidian, but also for several other varieties, especially Apache tears);  Glass Buttes, in northeastern Lake County, Oregon (photographs of fire, gold sheen, mahogany and rainbow obsidian from this locality are given in Pough, 1996);  Millard County, Utah (especially snowflake obsidian);  and Yellowstone National Park, Wyoming.  See also Weiner (1983).

REMARKS: The term obsidian is said to have resulted from a printer's error involving  the Latin word Obsianus, which was meant to allude to Obsius, who, according to Pliny, the Elder (Book xxxvi), discovered this rock in Ethiopia (formerly Abyssinia).

Obsidian is said to have been artificially decolorized -- see Nevada diamond under OTHER NAMES subheading.   Obsidian that exhibits flow lines has been cut into thin slices and cemented on to, for example, synthetic spinel,  to made doublets with the flow lines of the slices oriented in selected directions.  Some rainbow obsidian has been fashioned for pendants so it exhibits heart-shaped reflections;  these pieces are produced by cutting a groove from near the edge to about the center on the flat side of a suitably shaped piece such as a relatively thick cabochon.

Obsidian has been used as mirrors, for example, in telescopes. However, as noted by Pliny the Elder (ibid.), these so-called mirrors reflect silhouettes (umbras) rather than images like those seen in what most people call mirrors.  Pliny also reports that Caesar Augustus was the emperor who so-to-speak commissioned carving of  the obsidian elephants in the Temple of Concord.

The designation Apache tears is apparently based on American Indian lore:  These masses were said to have formed when Apache warriors leaped from a cliff to their death rather than being captured by an enemy.  Frederick Pough (ms) notes that "In Superior, Arizona, a cut along a cliff reveals thousands of staring black 'eyes' [the Apache tears], peeping out from gray flaky lids .... [And, he also notes that being] full of water, [these] marble-size nodules swell to baseball dimensions on being heated with this behavior giving expanded perlite,[which has] a role in building insulation." -- i.e., perlite is used as a light weight aggregate in such things as sound-absorbing tiles.

The Aztec term iztli was so-to-speak surnamed teotetl, which has been translated as divine stone, apparently in reference to its many uses -- Aztecs fashioned both tools and items for adornment from obsidian.  An especially noteworthy piece is the famous "monkey bowl," said to have been used in rituals involving human sacrifice, at one time exhibited in the National Museum of Anthropology in Mexico City, Mexico.

Items fashioned from basaltic composition glass, properly called sideromelane or tachylyte, are frequently called obsidian, especially in the marketplace.  Petrologically, different origins of their parent magmas and their different compositions distinguish basaltic glasses from obsidians.  So far as uses of basaltic glass as a gemrock, pieces made from them are extremely rare as compared to those made of obsidian.  Two things, in particular, appear to be responsible for this difference:  A.basaltic glasses are uncommon whereas obsidian is relatively common;  and   B.baslatic glasses, unlike obsidians, tend to dissolve readily in acids.  Webster (1994, p.290), however, does record cabochons fashioned from different colored basaltic glasses from the vicinity of Flinders River, northern Queensland, Australia.  

SIMULANTS:  

***Emerald obsidianite (also called Mount St. Helens obsidian) - a man-made glass that has been made to be sold as a souvenirs of the May 1980 eruption of Mount St. Helens, Washington. - [Appearance suffices.].

Glass portions of impactites -- [for pieces in,  for example, small carvings, some of these could be distinguished on the basis of chemical analyses or even microscopic means;  others might be nearly impossible to distinguish from obsidian unless their occurrence is known].

***Glassy slag - [for some -- determination of total composition is necessary, and this requires use of relatively sophisticated equipment].

***Helenite - glass said to have been produced by melting volcanic ash and/or dust erupted by Mount St. Helens, Oregon in 1980, that has been molded into, for example, subspherical and egg-shaped masses and sold for use as paper weights and curios. The purported origin of the material used to make at least some of these glass pieces has, however, been questioned by Nassau (1986). [For all of these I have seen appearance suffices.  By the way, Nassau's conclusions about the origin of the precursor material for this simulant are based on chemical analyses.]. 

***Man-made glasses of diverse compositions, including glassy slags from some smelters. Some bluish slags resemble hyalite opal -- e.g., some of those from the old iron furnace in the area now set aside as part of the Layette State Park, on Big Bay de Noc, Delta County, Michigan, where they occur as cobbles and pebbles on Slag Beach (I collected some of these for a lapidary friend in the early 1970s.). Several years later, I read that an apparently similar slag from Sweden is being fashioned into, for example, attractive cabochons (Johnson and Koivula, 1999) - [Some, but not all, of these are not distinguishable as manufactured without laboratory investigation.]   Along this line, most of the glass usually referred to as "Beach Glass" is Man-made glass.  In any case, the North American Sea Glass Association has recently held a annual contest that makes awards for to collectors for some of the diversely colored pieces of glass that have been given their current characteristics -- e.g., smooth edges -- as a result of erosion they have undergone within oceans, the Great Lakes, etc.   .

Tektites -- [same remarks as those given for "Glass portions of impactdites].  See TEKTITE entry.

Note: This listing raises another topic of at least tangential interest:  A number of green gemstones, many of which have been faceted, have been marketed as green obsidian.  Johnson, Reinitz and Owens (in Moses, Reinitz and McClure, 1998), however, report  that "We cannot recall seeing an example of transparent 'green obsidian' that has ever been proved to be a natural obsidian."

REFERENCES: Montgomery, 1981; Pough, ms in preparation, 1997;  VanLandingham, 1962, pt.2;  Weiner, 1983.
 

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