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.

This page will be revised and expanded periodically - a final version will eventually be published in the International Association for Obsidian Studies Bulletin. 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)

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).

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.

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.

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).

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).

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).

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 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.
• Rounded.
• Sub-rounded.
• Sub-angular.
• Angular.
• Not Determinable. Sample is too fragmental to determine roundness.

10. Shape: Sphericity

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.

THIS SECTION IS UNDER DEVELOPMENT

• Not Found.
• Very Rare.
• Rare.
• Occasional.
• Common.
• Abundant.
• Ubiquitous.

THIS SECTION IS UNDER DEVELOPMENT

• Not Known.
• Primary.
• Secondary.
• Colluvial.
• Fluvial.
• Glacial.
• Pluvial.
• Volcanic.

THIS SECTION IS UNDER DEVELOPMENT

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

1. Microlites

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

2. Crystallites

• Complete
• Skeletal

• Diffusion Front
• Hydration Rim

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