Obsidian is a felsic, naturally occurring volcanic glass. It has 69-77% silica and is relatively high in alkalis (>7%) and low in iron and magnesium oxides.
This rock is mostly extrusive (with some intrusive) and forms from the rapid cooling of highly viscous, often rhyolitic, or granitic magma. The rapid cooling and high viscosity results in an amorphous solid or non-crystalline structure.
The term ‘obsidian’ may have come from Obsidius, a Roman explorer who discovered it in Ethiopia. Both Pliny the Elder (Gaius Plinius Secundus) and Pliny the Younger attest to this. However, this origin isn’t very certain since obsidian relates to the Latin words opsidianus or obsidianus, which describe the reflective quality of this rock.
This volcanic glass made prehistoric weapons like arrowheads, swords, daggers, and tools like mirrors, blades, knives, etc. Today, gemstone quality makes beads, necklaces, anklets, pendants, rings, earrings, etc. It can potentially make the sharpest surgical blades. Also, you can use it in landscaping or making ornamental items.
Learn more about obsidian rock. We will discuss its appearance and colors, composition, formation, uses, and where it is found or occurs.
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Quick facts and properties
- Rock name: Obsidian
- Pronunciation: /əbˈsɪdi.ən, ɒb-/
- Rock type: Igneous
- Origin: Extrusive or volcanic but can be intrusive when on sills or dike edges
- Category: Volcanic glass
- Alternative name: Hyalopsite
- Colors: Mostly jet black, black, and dark gray, but it can be blue, green, brown, golden, mahogany, peacock, purple, golden, brown-green, grey-green, yellow, orange, amber, red, etc. Clear varieties are rare.
- Fracture: Conchoidal (since it is a glass without minerals) like quartz, chert, or flint.
- Texture: Glassy or vitreous
- Luster: Vitreous
- Mohs hardness scale: 5-6
- Specific gravity (density): 2.30-2.58g to 2.58 (2.3-2.58g/cm3)
- Eruption or melting point: 700-900°F
- Cooling history: Fast on the Earth’s surface allows minimal mineral formation.
- Chemical composition: Felsic
- Refractive index = 1.48 -153
- Tenacity: Brittle
- Magnetism: Yes. Tiny crystals of magnetite inclusions, a magnetic mineral, make it weakly magnetic.
- Tectonic environment: They form on the convergent boundary, especially in the Andean-type, where the ocean plate is subducted under a convergent continental crust and in intracontinental hot spots and rifts.
- Associated rocks: Rhyolitic flows
- Equivalent: Chemically equivalent to crystalline rocks like rhyolite or granite
- Resembles: It resembles tektite, onyx (a mineral), chalcedony, jet, and schorl.
- Optical properties: It is transparent, translucent, or opaque except for the edges. Some may show chatoyancy if they have parallel inclusion or the Schiller effect.
What does obsidian look like?
Obsidian is a hard, brittle, natural volcanic glass or mineraloid with a conchoidal fracture. It is not a mineral or crystal. However, it is often considered a volcanic or an extrusive igneous rock.
Usually, obsidian is a rhyolite glass formed from rapid quenching viscous, silica-rich lava or magma. The quick cooling and high viscosity prevent crystal formation.
However, it can form from other acidic, siliceous, or silica-rich magma or lava. If it forms from non-rhyolitic magma, it will bear a prefix to indicate which magma type it is. For instance, you can have dacite-obsidian.
Besides obsidian, other natural glass examples are pumice, Pele’s hair, Pele’s tears, perlite, and pitchstone. Also, trachyte is a mafic (usually basaltic) volcanic glass. However, mafic magmas hardly form volcanic glasses.
Color, fracture, banding, luster, Mohs hardness scale, and general appearance should help you identify obsidian.
Usually, obsidian is jet black, dark gray, or brownish, but it can be other colors. This volcanic glass has a vitreous luster and conchoidal fracture with a uniform, smooth isotropic (identical in all directions) structure and will not scratch glass.
It is mostly opaque with translucent edges and may show flow banding unique colors, some wavy or swirly. However, it may be translucent or rarely transparent. These properties should help you identify real and fake ones.
Also, obsidian resembles pitchstone, but the latter has a resinous luster, is not glassy, and is often a hackly fracture not conchoidal but can also be conchoidal. Also, it resembles the black, green-gray, or brown gravel-sized tektites from meteorite impacts.
Let us explore colors, banding or layering, and metastable state. It will further broaden your understanding of this volcanic glass.
1. Colors
Obsidian is usually pure or jet black, dark gray, or dark brownish in color. However, depending on the inclusions, it may have rare colors like tan, brown, golden, reddish, bluish, purplish, greenish, greenish-brown, or yellowish.
Also, this rock may show fancy coloration, swirls, and sheens like mahogany, flame, pumpkin, rainbow, silver sheen, lizard skin, snowflakes, midnight lace, golden glow, etc. See more on obsidian types and varieties.
These colors result from the refraction of light by crystallite and microlite refraction and other microscopic or nano-inclusion inclusions or bubbles.
These inclusions include magnetite, iron oxide, biotite, hornblende, and feldspar. For instance, inclusions like iron oxides like hematite or limonite can produce reddish or brownish variants.
Note: Only very few obsidians with few opaque inclusions or crystallites are colorless or clear. Most of the transparent or translucent varieties sold online are fakes, i.e., they are artificial glasses made in Indonesia and other places.
2. It shows banding or layering
Obsidian often shows banding with unique colors or appearance, with some showing swirls or wavy appearance. It results from the deformation of some crystallites or microlites by shear or strain during flow.
Usually, the high viscosity prevents magma from proper mixing, and each layer, swirl, streak, or line represents magma with unique composition color or other properties.
These bands develop parallel to the flow direction but may get distorted during flow on the surface.
3. It has a metastable state
It is important to note that obsidians occur in a metastable state. Therefore, these volcanic glasses will devitrify or form fine-grained grains by rearrangement of silica into tiny crystals, some forming snowflake-like crystals.
Most of those older than Miocene (23.03-5.333 Mya) are devitrified. The oldest known are Craterous (145-66 Mya) welded tuff and partially devitrified Ordovician (485.4-41.6 Mya).
Obsidian composition
Let us now look at the chemical and mineral composition of obsidian.
1. Chemical composition
Obsidian is considered a felsic rock, although it is technically a mineraloid. This volcanic glass is high in light elements like silicon, aluminum, oxygen, and sodium and low in mafic elements like magnesium or iron. Also, it is low in calcium.
Analyzing data from McCall (2005, p.269), the average wt.% chemical composition of obsidian from seven samples is SiO2: 74.84%, TiO2: 0.14%, Al2O3: 13.38%, Fe2O3: 1.33%, FeO: 0.69%, MnO: 0.085 %, MgO: 0.20%, CaO: 0.96%, Na2O: 3.97%, K2O: 4.33%, H2O: 0.52%, and P2O5: 0.14%.
Note: Some samples didn’t have weight composition determined. We used an average of those present.
For the above data and others we looked at, I can say that obsidian has > 69% silica, > 12% alumina, with combined K2O and Na2O at least 7%. However, this rock is low in magnesium, iron, and calcium oxide. Its chemical composition is similar to rhyolite or granite.
Lastly, this volcanic glass will have less than 1% water. However, it slowly hydrates and forms perlite, which is duller in luster.
2. Mineral composition
According to the International Union of Geological Sciences (IUGS), obsidian is a mineraloid, not a rock or mineral, since it has over 80% glass, an amorphous solid. However, it may have microscopic minerals of feldspar, hematite, magnetite, hornblende, plagioclase, and pyroxene, as we will see.
However, it does have some minor amounts of crystallites (minute to microscopic or embryonic crystals) and impurities.
On a thin section observation under a microscope, obsidian may show consistent clear glass, bubbles, or microlites and crystallites that formed earlier.
Crystallites and microlites may be rod-shaped (belonites), rounded (globulites), and hair-like twisted or coiled (trichites) bodies. Some are fused into chains, while others are parallel.
On the other hand, bubbles may be rounded, teardrop-shaped, needle-shaped, or shaped like torpedoes with some arranged parallel. Parallel bubbles and the various crystallites or microlites give obsidian some of the much-sought sheens.
These crystallites or microlites in obsidians usually include feldspar, hematite, magnetite, hornblende, plagioclase, pyroxene, etc.
Besides bubbles, crystallites, or microlites, obsidian glasses may have phenocrysts, but on very rare occasions. For instance, some vitrophyre samples have quartz, hornblende, or biotite phenocryst. Also, Rózsa, et. al (2003) findings showed the presence of zircon, chalcopyrite, pyrite, pyrrhotite, ca-poor, and rich orthopyroxene phenocrysts.
However, some phenocrysts, like chalcopyrite, pyrite, pyrrhotite, or Ca-rich orthopyroxene, may have been incorporated by hydrothermal activities.
How is obsidian formed, and where?
Obsidian forms from rapid quenching by water or air of highly viscous, silica-rich rhyolitic magma low in volatiles on or near the Earth’s surface.
However, it can also form as an intrusive rock on the edges of dikes, rhyolitic flows, plugs, necks, lenses, or sills. The cold country rocks will quickly quench magma and usually form the best quality obsidian without impurities, dirt, or ash.
What forms obsidian glass is 1) superfast cooling that gives crystals little time to grow and 2) highly viscous, silica-rich magma that impedes the movement of ions or atoms to form or grow crystals. Therefore, these magmas will form an amorphous or uncrystallized solid with only small amounts of crystallites, i.e., volcanic glass.
Usually, silica in this magma polymerizes (links together) into long chains early, increasing viscosity. This prevents the movement of molecules, atoms, or ions to form crystals or to nucleation sites. Also, it impedes magma flow.
The impeding of ion, atom, or molecule movement explains why some thick (a few hundred feet) blocky lava flows form obsidian glass, yet their thickness slows cooling.
Note that obsidian rocks often form during the last stage of effusive eruptions of the remnant, low-volatile, highly viscous (thick and pasty) magmas after initial explosive eruptions. Most will form blocky lava flow deposits or lava domes like the Obsidian Dome at June Lake, California.
Usually, the earlier explosive eruptions of crystalized rhyolite magmas will deplete most gases or volatiles, a reason for the low water content in obsidian rocks.
Lastly, magmas or lavas that form obsidian are likely trapped in a eutectic or nearly eutectic crystallization point, i.e., below the melting point of all its constituents. Thus, they will solidify or change the phase into amorphous solids or glass because they cannot form crystals with further cooling.
Where is obsidian found?
Obsidian rocks occur worldwide in areas with younger felsic or rhyolitic eruptions. Older ones are already devitrified.
Besides devitrification, these rocks are less common since they form from the most highly differentiated and evolved viscous rhyolitic or granitic magmas. Such have a small flow extent, and extrusion happens with difficulties.
Obsidian occurs in various countries, including Canada, Mexico, Argentina, Chile, Ecuador, Guatemala, El Salvador, and Peru in the Americas.
It occurs in Greece, Hungary, Iceland, Italy, Turkey, Georgia, New Zealand, and Scotland in Europe.
Australia, Kenya, Papua New Guinea, the Canary Islands, Armenia, Japan, and Azerbaijan are other places with obsidian.
In the US, obsidian occurs mostly in western states like Arizona, Idaho, Colorado, Utah, New Mexico, Wyoming, Washington, Oregon, and Texas.
Some specific locations with obsidian in the US include Mono–Inyo Craters (Panum Crater, Mono Crater) and Glass Mountain in California, Obsidian Cliff in Yellow Stone, Wyoming, and Black Spring in Utah. Also, this volcanic glass occurs in the Newberry complex, especially Big Obsidian Flow in Oregon.
Some locations in the US, like Glass Buttes, Black Spring, Glass Mountain, and Obsidian Cliff, have gemstone-quality obsidian deposits. Also, such qualities occur in Jalisco and Salar del Hombre Muerto in Mexico and Argentina, respectively.
What is obsidian used for?
Obsidian uses range from prehistoric weapons and tool making to jewelry and decorative ornaments. Also, it can help with landscaping and has a potential surgical medical use, and some people value these stones for metaphysical properties or uses.
Let us look at some of these uses and many others.
1. Making tools in the prehistoric period
Artifacts from Kariandusi (dating 700,000 years) to those in Lipari and all over Europe, Asia, the Middle East, and South, Central, and North America are evidence of the use of obsidian, chert, and flint to make stone tools.
These rocks were preferred due to their sharp, edged conchoidal fractures. Some of these tools were transported and traded over thousands of kilometers.
Obsidian rock made bladelets, axes, swords, daggers, polished mirrors, scrappers, arrowheads, mirrors, and other chipped tools in the Acheulian age (3.4 Mya to 2000 BC).
These tools were used as weapons in cutting umbilical cords, hunting, and circumcisions. Some still exist, like Aztec obsidian swords, mirrors, or Maya knives.
As recently as 1000 BC, Oceania traded these prestige-related tools. Also, they used it to inscribe rongorongo glyphs. Some people sell modern obsidian knives even today.
Lastly, being isotropic, this rock required skilled experts in knapping to make these tools.
2. Decorative ornaments
From prehistoric to modern times, obsidian has made various decorative ornaments, including sculptures and carvings.
For instance, Romans, Greeks, Aztecs, and others mined this rock to create decorative items like bowls, towers, mugs, plates, pendulums, chalices, spheres, statues, carvings, animal shapes, etc.
3. Archaeological significance
Archeologists can date obsidian artifacts using methods like radiocarbon, associated stratigraphic position and potsherds, hydration state (unreliable), or apply newer geochemical techniques.
Information from dating these artifacts can help reveal trade routes going thousands of kilometers and understand past societies.
3. It may contain minerals
Some obsidians may have lithophysae with acicular mineral crystals like clinoferrosilite and fayalite, which are valuable.
Clinoferrosilite occurs in Lake Naivasha in Kenya, Coso Mountains, and Obsidian Cliff USA, among other places.
On the other hand, fayalite is found in St. Peters Dome, Rockport, Massachusetts, Monroe in New York, and Iron Hill mine in Rhodes Islands.
4. Surgical medical usage
Obsidian rocks make knives and surgical scalpels (not FDA-approved) sharper than metallic blades, cutting only three nanometers thick like any other glass knife.
Also, these surgical scalpels may cause fewer inflammatory cells since they have smooth edges, i.e., they don’t have jagged edges like metallic knives when viewed on a microscope. However, they are brittle, making them potentially hazardous.
5. Semiprecious gemstone
Highly reflective, colorful, and gemstone-quality obsidian rocks are much sought by rockhounds and jewelers. Such are cut into cabochons, hearts, spheres, and other tumbled or polished. Clearer ones with few crystallites may be faceted or table-cut.
Uses of gemstone-quality obsidian rocks include making bracelets, earrings, rings, necklaces, anklets, pendants, brooches, etc. Also, they make black opal doublets or triplet composite.
However, the brittleness and lower hardness are not good for entirely obsidian rings or bracelets.
6. More uses
More uses include making audio turntable plinths, xeriscaping, fire pits, mulching, and garden decorations.
Frequently asked questions
The main difference between granite, rhyolite, and obsidian is crystal size or texture. Granite has a coarse-grained texture, rhyolite is fine-grained, and obsidian is mostly glassy with small amounts of crystallites.
These textures are influenced by cooling rate or depths, which is slow deep inside the earth’s crust for granite, faster near or on the surface for rhyolite, and rapid for obsidian. Otherwise, they all form from the same silica-rich rhyolitic magmas.
Prices or costs of gemstone quality obsidian range from US$1-10 per piece depending on its uniqueness, color, rarity, size, and other factors. However, uncut or raw obsidian will cost less than polished, tumbled, or cut.
On the other hand, the prices for non-gemstone quality will be US$ 200-300 per yard if you buy in bulk or about US$ 2-3 per pound for a smaller amount.
No. Rhyolite doesn’t present a health risk unless you inhale powders as you grind it, something that may cause silicosis, a common issue with any silica-rich rocks. However, its smooth, curved fracture forms a sharp edge intersection, sharper than a razor that can cut and injure you.
References
- McCall, G. J. H. (2005). Obsidian. In Selley, R. C., Morrison, C. L. R., & Plimer, I. R. (Eds.). Encyclopedia of geology (Vols. 1-5, pp.267-277). Elsevier Academic.
- Manutchehr-Danai, M. (2009). Dictionary of gems and gemology (3rd ed.). Springer.
- O’Donoghue, M. (ed.), (2006). Gems: Their sources, descriptions, and identification (6th ed.). Butterworth-Heinemann
- Bailey, R. A. (1989). Volcanic glass. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology. New York: Van Nostrand Reinhold.
- Miller, J. (2018, December 13). Obsidian. Oregon State University. https://volcano.oregonstate.edu/volcanic-minerals/obsidian
- Bonewitz, R. (2012). Rocks and minerals (1st ed.). DK Pub.
- E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.
- Okrusch, M., & Frimmel, H. (2020). Mineralogy: An introduction to minerals, rocks, and mineral deposits (1st ed.). Springer
- Anthony, J. W., Bideaux, R, A, Bladh, K. W., & Nichols, N. C., (Eds.) (2022). Handbook of mineralogy, Mineralogical Society of America, Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org/
- Rózsa, P., Elekes, Z., Szöőr, Gy., Simon, A., Simulák, J., Uzonyi, I., & Kiss, Á. Z. (2003). Phenocrysts in obsidian glasses. Journal of Radioanalytical and Nuclear Chemistry, 256(2), 329–337. https://doi.org/10.1023/a:1023957906156