Obsidian is a natural rhyolite volcanic glass that forms when silica-rich, highly viscous lava or magma rapidly cools. A natural volcanic glass is an amorphous solid without crystals. Most volcanic glasses, however, have a few tiny, microscopic, or submicroscopic crystals known as crystallites or microlites.
Non-rhyolitic silica-rich or acidic volcanic glasses will have a rock name like dacitic obsidian. Additionally, other volcanic glasses like pumice, sideromelane, Pele’s hair, Pele’s tears, trachyte, palagonite, apache tears, and pitchstone exist.
The origin of the name obsidian is unclear, but it is likely to be named after Obsidius, a Roman explorer who discovered this volcanic glass in Ethiopia. This name may also have originated from two Latin words, obsidianus or opsidianus, which describe the reflective nature of this glass.
This post will discuss obsidian, including its characteristics, appearance, composition, uses, and formation. It will also discuss the common varieties and types.
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Description and appearance
Obsidian is a hard, brittle, dark-colored, glassy rock with a conchoidal fracture and a Mohs hardness scale of 5–6. Its density is 2.3–2.8 g/cm3, and its refractive index is 1.48–153.
This natural volcanic glass is transparent, translucent, or opaque (except for the edges). It resembles jet, schorl, onyx, tektite, or chalcedony, but some specimens sold online are fakes.
The common obsidian colors are jet black, black to dark gray, and sometimes brown. These colors are caused by the interaction of light with crystallites and microlites of plagioclase, magnetite, hornblende, and biotite in a glass matrix.
However, some can have colors like yellow, golden, red, blue, green, or purple, along with a few colorless ones. These colors are due to inclusions and impurities like gas bubbles, iron, hematite, and limonite. Additionally, some varieties show unique colors and sheens, like rainbow, mahogany, snowflake, or silver.
Colorless varieties have few crystallites or impurities, allowing them to transmit light.
More characteristics of obsidian are:
- It doesn’t usually have phenocrysts or porphyritic textures. A few may, however, have quartz, biotite, and hornblende phenocrysts, forming a vitrophyre texture. Most of the larger crystals in specimens form later from hydrothermal fluid activities.
- It shows bands or layers, usually parallel to the flow direction, but some are distorted. These bands, or layers, form when different lava or magma portions flow but don’t mix well, resulting in variations in compositions, colors, and properties.
- Obsidian is in a metastable state, meaning, it is not in its least energy state. It will, therefore, devitrify to a more stable form with tiny silica crystals. Most Miocene or older deposits have undergone devitrification.
Chemical composition
Obsidian is an acidic rock with a composition resembling rhyolite or granite. It is rich in lighter-colored or felsic elements such as silicon, oxygen, sodium, aluminum, or potassium, and low in dark-colored or mafic elements such as calcium, iron, and magnesium.
Considering the average percentage composition of its various constituents, obsidian has more than 69% silica, more than 12% aluminum oxide, and at least 7% combined sodium and potassium oxides.
This volcanic glass is, however, low in calcium, magnesium, and iron oxides. Other minor constituents are phosphorus pentoxide, titanium oxide, and water, which is less than 1%.
Although the water content is less than 1%, this volcanic glass will slowly hydrate, forming perlite with a duller luster.
Mineral composition
Strictly speaking, obsidian is a mineraloid, not a rock, since it has over 80% glass. The IUGS recommends using the term ‘mineraloid’ for rocks containing more than 80% glass. You can, however, still call it a rock or volcanic glass because it forms when magma solidifies.
Besides glass, obsidian may have microscopic or embryonic crystals called crystallites or microlites. These include feldspars, hornblende, pyroxene, hematite, and magnetite. This rock also has tiny impurities and gas bubbles.
What are the obsidian varieties or types?
Obsidian varieties or types have unique inclusions, including bubbles that form unique shapes and patterns. This distinguishes them from ordinary black, gray, and brown colors or rarer red, yellow, orange, and blue ones.
1. Sheen obsidian aka silver or gold sheen
Sheen obsidian displays a shimmering silvery or glistering golden iridescence when rotated under the light. This sheen forms when the nearly flat, elongated, or stretched gas bubbles in this rock reflect light.
Obsidians with a golden or silky silvery sheen are common in Mexico and Morocco.
2. Snowflake obsidian
Snowflake obsidian, also known as flowering or starry night obsidian, has eye-like or radial white spots called spherulites embedded on a black or dark gray matrix. However, some varieties, like those from Gyumishkoe, Armenia, have a reddish or brownish matrix.
The white snow-like blotches, or spherulites, form when glass partially devitrifies, forming cristobalite, while the parts that don’t devitrify remain dark or black.
Some of the places to find snowflake obsidian are Wyoming at Yellow Stone Caldera, Utah at Black Spring, and Oregon at Glass Butte in the US.
Note: The dark green snowflakes from Denmark are artificial, not natural, and formed from glass-making mistakes.
3. Fire obsidian
Fire obsidian is a rare and the most prized variety with light pink to reddish iridescence or fire-like bursts. It occurs in Glass Butte, USA, and iridescence arises from the inclusion of thin layers of magnetite whose width is similar to that of the wavelength of light.
4. Rainbow obsidian
Rainbow obsidian is another highly prized variety with a colorful iridescence of blue, green, red, gold, purple, and sometimes pink colors. These colors form when submicroscopic pyroxene, hedenbergite, and sometimes mica and feldspar reflect light.
Rainbow obsidian can be found near Davis Creek and Rainbow Mine in California, as well as Glass Butt in Oregon in the US. It also occurs at La Revoltosa Mine in Mexico.
5. Mohagany obsidian
Mohagany obsidian, also known as red ribbon black or mountain mahogany obsidian, has black and reddish-brown stripes, swirls, or splashes. Hematite inclusions in this natural glass are responsible for the reddish-brown swirls.
6. Chatoyant obsidian
Ca’s eye or chatoyant obsidian will reflect a narrow, concentrated, silky, or wavy movable sheen when cut en cabochon. This sheen, or optical phenomenon, results from the reflection of light by nearly parallel minute cavities in this volcanic glass.
7. Leaf green obsidian
This is a rare, green, or emerald-colored obsidian. Its color is from inclusions like iron and gas bubbles. Please don’t confuse this natural volcanic glass with the green artificial glasses and moldavites sold only by many vendors.
8. Midnight lace obsidian
Midnight lace obsidian has delicate, lace-like white and dark patterns, streaks, or bands. It forms during the initial stage of volcanic glass formation as lava slowly moves. It occurs in Glass Butte, Oregon, USA, and the Caucasus Mountains, Armenia.
9. Lightning bolt or spiderweb obsidian
This is a dark-colored, usually gray or black, obsidian variety with an intricate whitish pattern or veins that look like a spider web. The whitish web-like pattern forms when mineral-rich fluids deposit materials in cracks that form during the rapid cooling of lava.
10. Tri or Tripple obsidian
It has harmoniously blended red, black, or clear colors caused by the minute inclusions in this natural volcanic glass.
11. Pumpkin obsidian
This variety is also known as brownish obsidian. It has a mahogany, orangish, or reddish color with darker or beige veins or patterns caused by impurities or inclusion including iron.
12. Blue pearl obsidian
It is also known as velvet peacock or seafoam obsidian and has heaving blue, violet, pink, greenish, lavender, and golden pattern. It resembles a peacock’s tail when polished.
13. Peanut obsidian
This variety has chatoyant reddish-orange spherulites made of radiating feldspar fibers. It resembles marekanite, but the round spherulites aren’t dark and occur in Sonora, Mexico.
How does obsidian form?
Obsidian forms from the quick quenching of silica-rich, highly viscous rhyolitic magma or lava. The rapid quenching and high viscosity (polymerized magma) hinder nucleation and crystal growth, forming an amorphous glass.
It is also possible that the glass forms because the magma is at or close to the eutectic point. A eutectic point refers to a homogenous mixture at a temperature lower than all the constituents present. Such a solution will form an amorphous solid, as cooling cannot trigger crystallization.
Most obsidian forms during the effusive eruption phase after an explosive eruption depletes volatiles. However, this rock can also form at the margins of sills, dikes, and lenses where cooling is quick. Those formed at margins is usually of the best quality as it doesn’t have impurities, ash, or dirt.
Where is obsidian found?
Obsidian is found in many countries or locations worldwide with relatively young rhyolitic eruptions. Most of the older deposits have undergone devitrification.
It occurs as domes or on the margins of rhyolitic flows, necks, or plugs. Notable deposits in the US are Glass Butte and Big Obsidian Flow in Oregon, Obsidian Cliff in Wyoming, Glass Mountain and Mono-Inyo Craters in California, and Black Spring in Utah. Some of these deposits have gem-quality obsidian.
Obsidian uses
Obsidian was important in making tools throughout the Acheulian age, which lasted from 3.4 million years ago to about 2000 BC. It made axes, swords, mirrors, arrowheads, scrappers, daggers, bladelets, etc.
It was also valued in prehistoric times for making decorative ornaments, carvings, and sculptures. These ornamental items included statues, spheres, pendulums, towers, mugs, and plates, among many others. The Romans, Aztecs, and Greeks particularly valued these items.
Today, obsidian has a potential use in surgical scalpels. Some varieties are semiprecious gemstones, while others have valuable minerals. Other uses are xeriscaping, fire pits, mulching, decorating gardens, and audio turntable plinths.
Beyond these uses, obsidian has archeological significance. It helps trace ancient trade routes by dating various artifacts. See more on 8 modern and ancient obsidian uses.