The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Is a Variety of||Natural Glass|
|Varieties||Apache Tears, Fire Obsidian, Mahogany Obsidian, Rainbow Obsidian, Sheen Obsidian, Snowflake Obsidian|
|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|
|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.|
|Hardness||5; 6 for basalt glass.|
|Density||2.33-2.42; 2.70—3.0 for basalt glass.|
|Birefringence||Crystals included in obsidian may be birefringent.|
|Cleavage||None. Fracture conchoidal (best example of this type of fracture). Very brittle. Basalt glass may be splintery, brittle.|
|Transparency||Transparent to opaque|
|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.|
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.
This attractive material has a wide variety of appearances.
Snowflake obsidian, with spherulites of cristobalite, is widely used in jewelry as beads and cabochons.
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.
Mahogany obsidian has a reddish-brown color due to iron impurities.
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.
Sheen obsidian has a sheen produced by inclusions of gas bubbles.
Pele’s Hair is a light, string-like volcanic basalt glass that can become airborne.
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.
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.
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.