33.48-ct obsidian oval cabochon, Brazil. Photo courtesy of liveauctioneers.com and Jasper52.
Obsidian is the most common form of natural glass and occurs in many attractive varieties. Since prehistoric times, people have used this material to make jewelry and carvings as well as practical objects, like knives.
| 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 ~50% 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. Green, blue, and reddish stones (transparent) are very rare. 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 crystal inclusions 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: Mexico (banded and sheen varieties); Utah (“snowflake obsidian,” cabochon 30 x 40 mm). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Formed by volcanism, obsidian is considered a rock. It commonly occurs in large pieces, and lapidaries frequently cut them into cabochons, beads, and carvings. Faceted pieces tend to appear dark, except when cut in small sizes. These gems can make delicate jewelry stones.
Faceted obsidians, 10 cwt. Photo courtesy of liveauctioneers.com and Sabertooth Auctions.
Heat-sensitive obsidian has a hardness of 5-6 and brittle tenacity. As a result, cutting and wearing this material requires some care. However, the broken edges of these rocks are sharper than any steel knife. Cultures across the globe have made scalpels, arrowheads, knives, and scrapers from obsidians.
Ceremonial dagger, obsidian blade with a turquoise, coral, and shell mosaic overlay on the hilt, about 10.5 inches long. Mixtec culture, Mexico, ca 1200-1500 CE. On display at the de Young Museum, San Francisco, California. Photo by Peter D. Tillman. Licensed under CC By-SA 2.0.
Transparent obsidians are quite rare, as are green, blue, and reddish colors. Hobbyists will occasionally facet these pieces. Obsidians can also display cat’s eyes.
10k yellow gold ring with cat’s eye obsidian. Photo courtesy of liveauctioneers.com and Epic Auctions and Estate Sales.
Although typically dark and opaque, obsidians come in many varieties popular with jewelry enthusiasts and mineral collectors.
This popular variety gets its name from its inclusions of snowflake-like spherulites of cristobalite. Jewelry makers frequently use these stones as beads and cabochons.
Sterling silver and snowflake obsidian jewelry set. Photo courtesy of liveauctioneers.com and Leonard Auction, Inc.
Beginning hobbyists often enjoy working with these cores of unaltered glass in nodular shells of decomposed obsidian. Commonly found in the southwestern American states of Arizona, Nevada, and New Mexico, some of these pieces have been faceted. This variety name comes from a legend of the Native American Apache people.
Apache tears. Photo by Stephanie Clifford. Licensed under CC By 2.0.
A variety with reddish-brown color due to iron impurities.
Mahogany “blood” obsidian cabochon. Photo courtesy of liveauctioneers.com and Easterly Auction Company.
These varieties can show multiple brilliant colors due to inclusions of magnetite nano-crystals. The “fire” variety contains thinner layers of magnetite than the “rainbow” variety.
“Double heart” rainbow obsidian sculpture, 9½ x 3¼ x 3¼ inches, Mexico. Photo courtesy of liveauctioneers.com and Heritage Auctions.
A variety with sheen produced by inclusions of gas bubbles.
Teardrop pendant, silver-sheen obsidian wrapped in copper. Photo © Adornments by Mae. Used with permission.
Named after the Hawaiian goddess of volcanos and fire, this light, string-like volcanic basalt glass can actually become airborne. It can catch on trees and outdoor structures.
Pele’s hair. Photo by Thomas Tunsch. Licensed under CC By-SA 2.0.
Natural obsidians may contain some specific inclusions, such as crystallites, that help distinguish them from artificial glass. Due to the difference in their refractive indices (RI), these will usually stand out in high relief when examined under magnification. Obsidians may also have straight banding and needles, two more telltale signs of natural origins.
Although not a definitive test, checking for magnetic reactions may help confirm a glass piece as natural. Obsidians can have a moderate to very strong reaction.
Some commonly encountered natural gemstones that may be substituted or confused with obsidians are jet, chalcedony, and schorl (black tourmaline). However, standard gemological tests should distinguish them. The specific gravity range of both chalcedony and schorl overlaps with that of obsidian, but an RI test should separate them.
You might encounter artificial or manufactured blue, green, or red glass pieces passed off as natural obsidians with these rare colors. See our glass gem listing for more information.
The United States is a major source of gem-quality material. Arizona, Colorado, and California each have several productive localities. Utah is a major source of the snowflake variety.
Other notable American sources include the following:
Obsidian lithics (stone tools), found in Yellowstone National Park. Photo by the National Park Service/Jacob W. Frank. Public Domain.
Mexico yields an abundance of banded and sheen varieties.
Other notable producers including the following:
Obsidian bird-point arrowhead, from Japan, Jōmon period, 14,000-300 BCE. Photo courtesy of liveauctioneers.com and North American Auction Company.
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.
Since household dust has a hardness of 7, which exceeds that of obsidian, use a lint-free cloth for manual cleaning. Resist the urge to just scrub away dirt or grime from the surface of any gem or object.
Obsidians may contain inclusions that could expand when heated. Avoid using mechanical cleaning systems and stick to mild soap, a soft brush, and warm water.
For more recommendations, consult our gemstone jewelry care guide.
Fragment of an obsidian revetment with a carved ivy design, 7 x 7.9 cm, Early Imperial Roman, 1st century CE. Gift of J. Pierpont Morgan, 1917. Metropolitan Museum of Art, New York. Public Domain.
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