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.
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.
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:
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.
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.
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.
The textural surface appearance of a fractured surface of obsidian.
Any structure found within the glassy obsidian groundmass that is visible with the naked eye or a hand lens.
This nodule is from the Inman Creek source in western Oregon.
Describes the characteristics (and presence or absence) of the original outer surface of unworked obsidian and the morphology of any cortex that is present.
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.
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.
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.
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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.
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