Obsidian Rocks By Wayne Farley, Dec. 2017

FIRST PART...

OBSIDIAN (GemRocks, Ref-1) ( Fr- obsidienne/obsidiane; Ger- Obsidian/Feuerkiesel; Nor- obsidian; Rus-  )   


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:   

  • Agata Negra (black agate) - misnomer sometimes used in the marketplace. 
  • Apache-tears - dark gray to nearly black, pebble-sized glass nodules, most of which have greatest dimensions ranging between two and four centimeters, that occur as remnants within or weathered out of light gray perlite. (Perlite, apparently derived from obsidian as the result of hydration involving meteoric water, is a light gray rock made up of concentrically fractured fragments.)  Many apache-tears in the marketplace are from Maricopa and Pinal counties, Arizona. 
  • Banded obsidian - the banding commonly exhibits a flow-like appearance; some roughly banded obsidian has been incorrectly designated onyx obsidian.    
  • Black lava glass - descriptive name given to some obsidian. 
  • Colombianite (americanite) - a variety of obsidian from Cali, Colombia. 
  • Chatoyant obsidian - name sometimes applied to varieties that are iridescent in diverse colors; appearance is apparently due to the presence of minute inclusions and/or bubbles. 
  • Electric blue obsidian - obsidian with a vibrant blue color.  
  • Fire obsidian - iridescent obsidian from Glass Buttes, Oregon;  "fire" is said to be due to reflection of light off thin layers the refractive indices of which are higher than the rest of the obsidian because of their containing extremely small magnetite crystals. See Fig-8. For additional pictures and information on fire obsidian, check the article “A Passion for Working Fire Obsidian” by Tom Dodge in “The Gem Association of Great Britain” 
  • Flame obsidian - another name of fire obsidian: (also called "mar-chett" according to Emory Coons, (p.c., 2008), who also notes that the one from Glass Buttes apparently owes its appearance to "a paper thin opaque line of red (mostly), sometimes yellow or greenish, or mix of the varying color. It is very rare to have what is called fire on it.  
  • Flowering obsidian - similar to snowflake obsidian. 
  • Glass (or glassy) lava - overall term that has been used, especially in the field. 
  • Glass agate - atrocious misnomer sometimes given obsidian. 
  • Golden sheen obsidian (also gold or golden obsidian) - name widely given in the marketplace to sheen obsidian with the predominant "sheen" a golden brown color.   
  • Iceland agate (or Iceland agate lava) - misnomer sometimes given to a brownish or grayish variety of obsidian from Iceland. 
  • Iris obsidian - another name for rainbow obsidian. 
  • Itatli - obsidian (Aztec). 
  • Iztli - obsidian (Aztec). 
  • Lassenite - glass of trachytic composition (trachyte is the aphanitic equivalent of syenite, which is composed of 90 or more percent of alkali feldspar) from the vicinity of Lassen Peak, California.
  • Libyan glass - see Desert glass.
  • Mahogany obsidian (also Mountain mahogany) - reddish subtranslucent obsidian, commonly including black or gray band-like or streaked swirl-like patterns. Some of the most notable sources of this stone today include South America, the United States, Afghanistan, Mexico, and Japan. 
  • Marekanite - name for Apache tear-like masses OR, according to some ambiguous statements, a mottled brown and black obsidian from the vicinity of the Marekanka River, which flows into the Sea of Okhotsk, off eastern Siberia.
  • Marskanite - name applied variously -- e.g., to mottled brown and black obsidian from Siberia; to any cloudy, smoky gray obsidian; and to brown and gray, commonly in part yellowish or reddish, obsidian (especially those from Mexico). 
  • Mexican 'Mayan' - see Rainbow obsidian. 
  • Mount Saint Helens emerald - a misleading misnomer given obsidian (and even more often to a green glass produced by melting ash erupted by Mount St. Helens. 
  • Montana jet - a misleading misnomer given some black obsidian from the Yellowstone National Park area, Wyoming. 
  • Mountain mahogany - See Mahogany obsidian. 
  • Nevada diamond - term that has been applied, albeit rarely, to what is said to be artificially decolorized obsidian. 
  • Obsidian "cat's-eye" (or"cat's eye obsidian) - name sometimes given to obsidian that has a golden chatoyant-like appearance; see Golden sheen obsidian. 
  • Obsidianite - name introduced by Walcott (1898) for what are now known to be tektites (australites) from Australia; however, according to Bates and Jackson (1987) this term came to have wider usage, and "Most stones originally described as 'obsidianite' were later shown to be true obsidian and not tektites." [To date, however, I have not found examples to document this purported later, wider usage.] 
  • Onyx obsidian - term sometimes applied to banded obsidian. 
  • Peanut obsidian - a gray to greenish gray perlite that contains red or brownish red stellate spherulites, which consist of radiating fibers of hematite-stained feldspar within shells of chalcedony;  the overall appearance -- both color- and size-wise -- roughly resembles a mass of peanuts.  A noteworthy occurrence is near Alamos, State of Sonora, Mexico. 
  • Pearlylite - name sometimes given in the marketplace to obsidian in jewelry. 
  • Pumpkin obsidian -obsidian with a pumpkin-orange color. 
  • Porphyritic obsidian - obsidian containing sporadic phenocrysts. 
  • Rainbow obsidian (also rainbow sheen obsidian and iris obsidian) - obsidian that exhibits a multicolored iridescence, apparently because of the presence of inclusions --e.g., that from Glass Buttes, Lake County, Oregon and that, called Mexican 'Mayan'  from the state of Jalisco, Mexico (see Koivula, Kammerling and Fritsch, 1993).    

  • WILL BE CONTINUED NEXT MONTH......