OBSIDIAN: Mexico (banded and sheen varieties): Utah (“snowflake obsidian,” cabochon 30 x 40 mm). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Obsidian Information
| Data | Value |
|---|---|
| Name | Obsidian |
| Is a Variety of | Natural Glass |
| Varieties | Apache Tears, Fire Obsidian, Mahogany Obsidian, Rainbow Obsidian, Sheen Obsidian, Snowflake Obsidian |
| Formula | Variable composition: SiO2 approximately 66-72% + oxides of Ca, Na, K, and so forth. Basaltic glass is ~5O% SiO2. |
| Etymology | After Obsius, an explorer who discovered this material in Ethiopia, according to the Ancient Roman naturalist Pliny the Elder. |
| Occurrence | Obsidian is found in areas of present and former volcanic activity. |
| Inclusions | Elongated, torpedo-shaped bubbles, round bubbles, teardrop-shaped bubbles. Bubbles are often in parallel arrangement. Needlelike inclusions may give a silvery sheen. Protogenic silica minerals crystallizing in obsidian may be white and resemble snowflakes, hence the term snowflake obsidian. |
| Colors | Black; gray, banded with brown streaks. Iridescence noted: gold, silver, blue, violet, green, and combinations of these colors, due to inclusions of minute bubbles that reflect light. |
| Fracture | Conchoidal (best example of this type of fracture).Basalt glass may be splintery |
| Hardness | 5; 6 for basalt glass. |
| Cleavage | None |
| Wearability | Poor |
| Crystallography | Amorphous; usually as rounded masses ejected in volcanic eruptions, as small broken pieces, fine, hairlike filaments (for example, Pele’s Hair), and as flows. |
| Refractive Index | 1.48-1.51; usually 1.49 |
| Birefringence | None, but crystals included in obsidian may be birefringent. |
| Luminescence | None. |
| Luminescence Present | No |
| Pleochroism | None. |
| Optics | Isotropic. |
| Luster | Vitreous. |
| Polish Luster | Vitreous. |
| Specific Gravity | 2.25-3.00. (Typically 2.33-2.42; 2.70-3.00 for basalt glass). |
| Transparency | Transparent to opaque. |
| Phenomena | Iridescence, chatoyancy. |
“Obsidian,” showing conchoidal fractures, by Kevin Walsh is licensed under CC By 2.0
Comments
Obsidian is naturally occurring volcanic glass. This material is considered a rock. It’s heat sensitive and brittle, so cutting requires great care. Faceted gems tend to be very dark, except in small sizes. These pieces make delicate jewelry.
Varieties
This attractive material has a wide variety of appearances.
Snowflake obsidian, with spherulites of cristobalite, is widely used in jewelry as beads and cabochons.
“Snowflake Obsidian” by James St. John is licensed under CC By 2.0
Apache tears, which are cores of unaltered glass in nodular shells of decomposed obsidian, are popular among beginning hobbyists. Some of these have been faceted.
“Apache Tears” by Stephanie Clifford is licensed under CC By 2.0
Mahogany obsidian has a reddish-brown color due to iron impurities.
“Mahogany Obsidian” by James St. John is licensed under CC By 2.0
Rainbow and fire obsidian show multiple brilliant colors due to inclusions of magnetite nanocrystals. Fire obsidian contains thinner layers of magnetite than the rainbow variety.
“Fire Obsidian” by Jessa and Mark Anderson is licensed under CC By 2.0
Sheen obsidian has a sheen produced by inclusions of gas bubbles.
Obsidian has been knapped into arrowheads and other tools since prehistoric times. This material can be shaped into a blade with a cutting edge sharper than surgical steel. “(HMM) Ancient technology – a broken piece of obsidian stone tool” by aotoro is licensed under CC By 2.0
Pele’s Hair is a light, string-like volcanic basalt glass that can become airborne.
Pele’s hair can be found caught in trees and outdoor structures. “Pele’s hair (DSC_3905)” by Thomas Tunsch is licensed under CC By-SA 2.0
Synthetics
Artificial or manufactured glass in blue, green, or red is sometimes passed off as obsidian. Natural obsidian does not form in these colors. (However, these colors can be shown through iridescence). See the glass gem listing for more information.
Sources
The United States is a major source of gem-quality material.
- Oregon: fire, mahogany, and rainbow varieties are known.
- Wyoming: notably at Yellowstone National Park.
- New Mexico: Apache tears.
- Arizona; Colorado; California; Nevada: several localities.
- Utah: major source of snowflake variety.
- Hawaii: Pele’s hair and other varieties
- Mexico: abundant, especially banded and sheen varieties.
- Ecuador; Iceland; Indonesia; Italy; Japan.
Stone Sizes
Fragments range from microscopic to many inches across. Carvings up to 8-10 inches could be made. Larger pieces are available in place in certain localities.
“Obsidian, black pendant, copper wire wrapped” by D Fredericks is licensed under CC By 2.0
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