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Obsidian Terminology

WHAT IS OBSIDIAN? | TERMINOLOGY: MEGASCOPIC | TERMINOLOGY: MICROSCOPIC

WHAT IS OBSIDIAN?
Obsidian is a naturally-occurring volcanic glass and is probably one of the most widely recognized rocks or minerals. The name obsidian is one of the most ancient of rock names still in use today and was brought into the language by Pliny (the Elder) almost two millenia ago. Iddings (1888:261) writes that the stone was named after "...Opsius, its discoverer, in Ethiopia, according to Pliny, who says that when laid in chamber walls in the form of mirrors it reflects shadows instead of images."

The term obsidian is a textural one and the chemical composition of the glass can vary from basaltic to rhyolitic. Since the obsidians of varying compositions are often megascopically indistinguishable, they are all lumped into this one category. If the composition is known, however, the term obsidian should be preceded by the appropriate rock name as defined by silica content, i.e. basaltic, andesitic, dacitic or rhyolitic. Though the composition of obsidians is variable, the vast majority are rhyolitic in composition, the reason for this being related to the high viscosity of high-silica melts.

Wikipedia on Obsidian

DESCRIPTIVE TERMINOLOGY: MEGASCOPIC
COLOR | LIGHT TRANSMITTANCE | SURFACE LUSTER | SURFACE TEXTURE | INCLUSIONS | CORTEX | MAXIMUM DIMENSION | SHAPE | ABUNDANCE | CONTEXT

The following set of descriptive terms and definitions were developed to provide a set of baseline standards that could be used to describe visual characteristics obsidian artifacts and nodules of raw materials. The terms have been adapted from different sources, particularly those of sedimentary petrology.

Megascopic attributes are defined here as characteristics that can be determined with the naked eye or with a hand lens up to about x15 magnification.

These attributes can serve as an indirect reflection of different geologic and geomorphic processes. Light transmittance and surface luster, for instance, indirectly indicate the degree of crystallinity of the glass. Similarly, the shape, sphericity, and cortex morphology of nodules can provide clues about the depositional and transport environments. Some of these same attributes are also discussed by Phillip H. Shelley (1993. A Geoarchaeological Approach to the Analysis of Secondary Lithic Deposits. Geoarchaeology 8:59-72) in the context of the description of secondary deposits of lithic materials.

Any comments, clarifications, or suggestions by readers can be sent to Craig Skinner and will be greatly appreciated.

1. Color: Hand Specimen

Color(s) of a wet opaque hand sample of obsidian. The Geological Society of America Rock-Color Chart is considered the color standard by which obsidian colors will be assigned. Colors should be described using Munsell color values for hue, value, and chroma (e.g. 5G 5/2) and by standardized color names (e.g. black, medium gray, greenish-black, etc.). When more than one color is present (as with mottled and other mixed color textures), the dominant color is listed first, with other minor colors listed in descending order of abundance. Any unusual colors should be described in narrative.

Not Applicable. Use in additional color fields when only one color is present or when a sample is too small or thin to determine the true color.

Some common obsidian colors (and their Munsell designations) include:

  • Black (N 1/0)
  • Grayish Black (N 2/0)
  • Dark Gray (N 3/0)
  • Medium Dark Gray (N 4/0)
The color of obsidian is related largely to the degree of crystallinity of the glass and the mineral phase of some of the crystalline components of the glass. Chemical composition is typically not affected by the color of the obsidian.

2. Color: Texture

Color texture describes the way in which the colors are distributed throughout the obsidian. The color texture is often best determined using a thin flake or edge. See Figure 1.

  • Not Applicable. Use when the presence of cortex or patina prevents the determination of this attribute.
  • Banded, Distinct. Banding occurs in distinct and easily definable bands with easily delineated borders; banding may be linear to curvilinear.
  • Banded, Indistinct. Banding can be seen but is often indistinct with "fuzzy" boundaries and little contrast.
  • Mottled. Colors occur in random patches; variegated.
  • Veined. Colors occur in a "mossy" pattern with thin dendritic stringers identifiable; glass may appear as almost cloudy.
  • Uniform. The color is consistent or nearly so throughout the sample.
  • Other. Other color texture (describe in comments).
Color texture terminology is from Adams (1980) and Skinner (1983 and 1987). Banding that is clearly visible in a thin obsidian flake is often undetectable in a hand sample of glass.


Figure 1. Color texture guide for obsidian.

3. Light Transmittance

Refers to the degrees of transparency (clarity) or light transmittance qualities of a thin obsidian flake or edge (approximately 1 mm in thickness) when viewed with a 60 watt incandescent light.

  • Not Applicable. Use when the presence of cortex or patina prevents the determination of this attribute.
  • Opaque. Little to no light passes through the glass.
  • Translucent. Light passes through the glass but letters and numbers are obscured and cannot be read.
  • Transparent. Light passes through easily and letters or numbers can be easily read.
The degree of light transmittance is most often a direct reflection of the degree of crystallinity of the glass.

4. Surface Luster

The luster or quality of light reflected from a clean and patina-free fractured surface of obsidian. If the surface luster is variable (as it often is with banded glass), the luster should be recorded for the only for the glassiest portion of the surface.

  • Not Applicable. Use when the presence of cortex or patina prevents the determination of this attribute.
  • Adamantine. Having a hard, brilliant luster like that of a diamond.
  • Chatoyant *. The surface exhibits a pearl-like sheen or iridescence. Sometimes referred to as pearlescence or opalescence. A movable wavy or silky sheen is concentrated in a narrow band of light that changes its position as the glass is turned.
  • Earthy *. A lack of luster produced by a surface that scatters light; matte and grainy surface textures often exhibit an earthy luster.
  • Greasy. Looking as if covered by a thin layer of oil.
  • Resinous *. Having the appearance of resin.
  • Vitreous *. A glassy texture with the luster of freshly broken window glass.
Most of the surface luster attributes of obsidian are highly dependent on the degree of crystallinity of the glass. A chatoyant luster is the result of the presence of very small (5-20 micron diameter) bubbles in the glass. Standard mineral surface luster terminology is primarily from Dana (1959) and has not been modified for use with obsidian. The types of luster most often encountered in obsidian are marked with an asterisk (*).

5. Surface Texture

The textural surface appearance of a fractured surface of obsidian.

  • Not Applicable. Use when the presence of cortex or patina prevents the determination of this attribute.
  • Smooth. Smooth and shiny surface similar to that of a broken piece of window glass.
  • Flawed. Small flaws are visible on the surface of the otherwise smooth glass (see Inclusions, microphenocrysts). A hand lens may be helpful in distinguishing this texture.
  • Matte. Surface has the appearance of a piece of matte paper; the surface is dull but individual phenocrysts cannot be distinguished with the naked eye.
  • Grainy. The surface has a decidedly grainy or sugary appearance though the grains may be quite small.
  • Hackly. Poor quality glass - surface may be very irregular when fractured. Rarely of artifactual quality. Easily visible phenocrysts are often present. The scale of the irregularities may range from those easily visible with the naked eye to those distinguishable only with a hand lens.
  • Other. Other surface texture is present (describe in comments).
The surface texture primarily reflects the degree of crystallinity of the glass. Glassy obsidian tend to be darker in color (most often black) than more crystalline obsidian (often gray). Surface texture terminology is from Adams (1980) and Skinner (1987).

6. Inclusions

Any structure found within the glassy obsidian groundmass that is visible with the naked eye or a hand lens.

  • Not Applicable. Use when the presence of cortex or patina prevents the determination of this attribute.
  • None. No inclusions in the glass.
  • Accidental. Accidental inclusion; a foreign inclusion in the glass (if genetically related to the glass, an autolith - if unrelated, an accidental inclusion or xenolith.
  • Bubbles. Bubbles in the glass that are visible to the naked eye.
  • Microphenocrysts. Phenocrysts too small to be discernable with the naked eye. Megascopically, their presence is indicated by a "flawed" appearance of a fractured surface of glass (a porphyritic texture). A hand lens may be necessary to distinguish the presence of microphenocrysts.
  • Megascopic Phenocrysts. A crystal visible in the glassy obsidian groundmass (a porphyritic texture).
  • Spherulites. Spherical mass of acicular crystals radiating from a central point. Typically occur singly or in aligned "trains".
  • Other. Other types of inclusions are present (describe in comments).

Small flaws in glassy surface typical of presence of very small crystals (microphenocrysts).
This nodule is from the Inman Creek source in western Oregon.

7. Surface Cortex

Describes the characteristics (and presence or absence) of the original outer surface of unworked obsidian and the morphology of any cortex that is present.

  • Not Applicable. Cortex is absent and cannot be distinguished.
  • Cortex Presence or Absence. Is any original cortex present?
  • Cortex Present, Smooth. The cortex is relatively smooth and may be quite dull in appearance due to physical weathering, hydration, or the presence of patina. The surface cortex of young autobrecciated obsidian flows often exhibits few signs of weathering and can be easily confused with the interior surface of a culturally modified artifact. When suspected, the possibility of smooth young flow surfaces should be noted as a comment.
  • Cortex Present, Crenulated. The surface is covered by small fingernail-shaped (curved) grooves and arc-shaped chips that vaguely resemble the worm trails found in driftwood.
  • Other. Other type of cortex (describe in comments).
  • Percentage. Estimated percentage of cortex found on dorsal or ventral surface of specimen.
The type of cortex found can provide indications about the geomorphic processes involved in the transport of glass to secondary contexts and the distance traveled. Obsidian with smooth cortex is most often associated with flows, domes, and short transport distances. Obsidian with crenulated cortex is typical of glass that has been fluvially transported along gravel beds - the pitting is due primarily to mechanical abrasion of the glass (see Kuenen 1956).

8. Maximum Dimension

The maximum measurable dimension of the artifact in centimeters, accurate to the nearest millimeter.

The maximum dimension of an artifact is sometimes a direct reflection of the size of the available lithic material. When the maximum dimension of a potential source is exceeded by the maximum dimension of an artifact (after taking into account attrition due to reduction and manufacture), the source may be eliminated. The use of this attribute to exclude sources of obsidian from consideration may reliably used only after the geologic source area has been thoroughly investigated, however.

9. Shape: Roundness

The generalized roundness of an intact or nearly intact nodule of obsidian and is generally best applicable to the description of raw material. Roundness standards are from Powers (1953) and Pettijohn (1975). See Figure 2.

  • Not Applicable. Use when no indication of roundness of original raw material is present (as with most obsidian tools).
  • Well-rounded. No original faces, edges, or corners are left; no secondary corners are present.
  • Rounded. Original faces are almost completely destroyed; all original edges and corners have been smoothed to rather broad curves and original faceshave been almost completely destroyed by abrasion.
  • Sub-rounded. Partially rounded, showing considerable but not complete abrasion; original form is still evident but edbes and corners are rounded to smooth curves.
  • Sub-angular. Somewhat angular; free from sharp edges but not smoothly rounded; shows signs of abrasion but retains original form..
  • Angular. Sharp edges and corners and little evidence of abrasion.
  • Very Angular. Reserved for nodules whose edges are sharp enough to cut.
  • Not Determinable. Sample is too fragmental to determine roundness.
The roundness of the nodule is most applicable to unmodified raw materials or large fragments of nodules. Nodule shape is highly dependent on the geomorphic transport method and distance (when found in secondary deposits) and the emplacement style (flow or dome).


Figure 2. Roundness guide for obsidian nodules (modified from Powers 1953).

10. Shape: Sphericity

Describes the relationshiop to each other of the various diameters (length, width, and thickness) of a nodule. The generalized sphericity of an intact or nearly intact nodule of obsidian and is generally best applicable to the description of raw material. Sphericity standards are from GSA Data Sheet 18.1 (also see Pettijohn 1975). See Figure 3.

  • Not Applicable. Use when no indication of roundness of raw material is present (as with most obsidian tools).
  • Discoidal.
  • Sub-discoidal.
  • Spherical.
  • Sub-prismoidal.
  • Prismoidal.
  • Not Determinable. Sample is too fragmental to determine roundness.
The sphericity of the nodule is most applicable to unmodified raw materials or to large fragments of nodules. Nodule shape is highly dependent on the geomorphic transport method and distance from source (when found in secondary deposits).


Figure 3. Sphericity guide for obsidian nodules (modified from Powers 1953 and MacLeod 2002).

11. Abundance

THIS SECTION IS UNDER DEVELOPMENT

  • Not Found.
  • Rare.
  • Occasional.
  • Uncommon.
  • Common.
  • Abundant or abundant.
  • Ubiquitous.
12. Context: Primary or Secondary

THIS SECTION IS UNDER DEVELOPMENT

  • Not Known.
  • Primary.
  • Secondary.
  • Colluvial.
  • Fluvial.
  • Glacial.
  • Pluvial.
  • Volcanic.
13. Context: Transport Processes

THIS SECTION IS UNDER DEVELOPMENT

  • Animal.
  • Human.
  • Mass-Wasting.
  • Fluvial.
  • Glacial.
  • Pluvial.
  • Volcanic.
    • Ash-Flows
    • Lahars
    • Tephra

DESCRIPTIVE TERMINOLOGY: MICROSCOPIC
MICROLITES | CRYSTALLITES | HYDRATION RIMS

The following set of descriptive terms and definitions were developed to provide a set of baseline standards that could be used to describe microscopic characteristics obsidian artifacts and raw materials. The terms have been adapted from numerous sources, particularly those of microscopic petrography. Although microscopic petrographic attributes cannot generally be used to characterize individual sources, they can sometimes be used to corroborate the results of trace element provenience studies. When artifacts have been prepared for obsidian hydration measurements, these microscopic characteristics of obsidian can be easily observed.

THIS SECTION IS UNDER DEVELOPMENT

1. Microlites

  • Clavalite
  • Cumulite
  • Glubulite
  • Longulite
  • Margarite
  • Prismatic Microlites
    • Acicular
    • Coiled or looped
    • Normal
  • Scopulite (Arborescent Microlite)
  • Spiculite
  • Trichites
    • Acicular
    • Asteroidal

Normal prismatic microlites x150 (Big Obsidian Flow, Oregon) - the rod-shaped crystals are about 10 microns long.

Asteroidal trichites x 150 (Cougar Mountain, Oregon).

2. Crystallites

  • Complete
  • Skeletal
3. Obsidian Hydration Rims

  • Diffusion Front
  • Hydration Rim

Obsidian hydration rim found on an Easter Island artifact

REFERENCES CITED
Adams, Rex K. 1980. Debitage Analysis: Lithic Technology and Interpretations of an Archaic Base Camp Near Moquino, New Mexico. Unpublished Master's Thesis, Department of Anthropology, Eastern New Mexico University, Portales, New Mexico.

American Geological Institute. No Date. AGI Data Sheets. American Geological Institute.

Bates, Robert L. and Julia A. Jackson. 1987. Glossary of Geology. American Geological Institute, Alexandria, Virginia.

Bowman, Kathleen K. 1987. An Analytic Study of Obsidian from the Middle Rio Puerco Valley, New Mexico. Unpublished Master's Thesis, Department of Anthropology, Eastern New Mexico University, Portales, New Mexico.

Clark, Bruce R. 1970. Stress-Controlled Orientation of Microlites in Obsidian. EOS 51(4):425.

Clark, Donovan L. 1961. The Application of the Obsidian Dating Method to the Archaeology of Central California. Unpublished Ph.D. Dissertation, Stanford University, Palo Alto, California.

Dana, James. 1959. Dana's Manual of Mineralogy, 17th edition revised by C.S. Hurlbut, Jr. John Wiley & Sons, Inc., New York, New York.

Goddard, E.N., Parker D. Trask, Ronald K. DeFord, Olaf N. Rove, Joseph T. Singewald, Jr., and R.M. Overbeck. 1980. Rock-Color Chart. Geological Society of America, Boulder, Colorado.

Haarklau, Lynn, Lynn Johnson, Dave Wagner, Richard E. Hughes, Craig E. Skinner, Jennifer J. Thatcher, and Keith Myhrer. 2005. Fingerprints in the Great Basin: The Nellis Air Force Base Regional Obsidian Sourcing Study. Report prepared by Prewitt & Associates, Inc., Austin, Texas, for Nellis Air Force Base, Nevada.

Heinrich, E. Wm. 1956. Microscopic Petrography. McGraw-Hill Book Co., New York, New York.

Heizer, Robert F., Howel Williams, and John Graham. 1965. Notes on the Mesoamerican Obsidians and Their Significance in Archaeological Studies. In Sources of Stones Used in Prehistoric Mesoamerica Sites, Contributions of the University of California Archaeological Research Facility 1: 94-103.

Iddings, Joseph P. 1888. Obsidian Cliff, Yellowstone National Park. U. S. Geological Survey Seventh Annual Report 3:249-295.

Johannsen, Albert. 1931. . The University of Chicago Press, Chicago, Illinois.

Kuenen, Ph. H. 1956. Experimental Abrasion of Pebbles: 2. Rolling by Current. Journal of Geology 64:336-368.

Landis, Daniel G. and Robert L. Sappington. 1985. Appendix J: Obsidian Sourcing Analysis. In A Cultural Resources Survey and Site Testing of the Bonneville Power Administration's Malin-Warner 240kV Transmission Line, Klamath County, Oregon, and Modoc County, California, edited by M.J. Rodeffer and Jerry R. Galm, pp. 537-558. Eastern Washington University Reports in Archaeology and History 100-36, Cheney, Washington.

MacLeod, N. 2002. Geometric Morphometrics and Geological Form-Classification Systems. Earth-Science Reviews 59:27-47

O'Keefe, John A. 1976. Tektites and Their Origin. Elsevier Publishing Co., New York, New York.

Pettigrew, Richard M. 1983. Archaeological Investigations at the Wagontire Site (35HA328), Harney County, Oregon. OSMA Survey Report 83-4, University of Oregon, Eugene, Oregon.

Pettigrew, Richard M. and Clayton G. Lebow. 1987. Data Recovery at Sites 35JA27, 35JA59, and 35JA100, Elk Creek Lake Project, Jackson County, Oregon. Report prepared for the U.S. Army Corps of Engineers by INFOTEC Research, Eugene, Oregon.

Pettijohn, F. J. 1975. Sedimentary Rocks. Harper & Row, Publishers, New York, New York.

Powers, M. C. 1953 A New Roundness Scale for Sedimentary Particles. Journal of Sedimentary Petrology 23:117-119.

Ross, Clarence S. 1962. Microlites in Glassy Volcanic Rocks. American Mineralogist 47:723-740.

Rutley, Frank. 1891. Notes on Crystallites. Mineralological Magazine 9(44):261-271.

Shelley, Phillip H. 1993. A Geoarchaeological Approach to the Analysis of Secondary Lithic Deposits. Geoarchaeology 8:59-72.

Skinner, Craig E. 1983. Obsidian Studies in Oregon: An Introduction to Obsidian and An Investigation of Selected Methods of Obsidian Characterization Utilizing Obsidian Collected from Prehistoric Quarry Sites in Oregon. Unpublished Master's Project, Interdisciplinary Studies, University of Oregon, Eugene, Oregon.

Skinner, Craig E. 1987. Lithic Database Project: Software Documentation. Unpublished manuscript in possession of the author.

Skinner, Elizabeth J., John L. Fagan, and Peter W. Ainsworth. 1989. Lithic Landscape: Technological Constraints of Size Shape and Amount of Cortex. Paper presented at the 47th Plains Conference, Sioux Fall, South Dakota, October, 1989.

Suzuki, Masao. 1973. Chronology of Prehistoric Activity in Kanto, Japan. Journals of the Faculty of Science, Unievrsity of Tokyo, Section 5 (Anthropology) 4(Part 3):241-318.

Williams, Howel, Francis J. Turner, and Charles M. Gilbert. 1954. Petrography: An Introduction to the Study of Rocks in Thin Section. W. H. Freeman and Co., San Francisco, California.

Zirkel, Ferdinand. 1987. Microscopic Petrography. Professional Papers of the Engineer Department, U. S. Army, No. 18, Report on the Geologic Exploration of the Fortieth Parallel, Vol. 6.

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Last Updated: 04/20/2016
Northwest Research Obsidian Studies Laboratory