These colors result from the refraction of light by crystallite and microlite refraction and other microscopic or nano-inclusion inclusions or bubbles. See more on obsidian types and varieties. However, depending on the inclusions, it may have rare colors like tan, brown, golden, reddish, bluish, purplish, greenish, greenish-brown, or yellowish.Īlso, this rock may show fancy coloration, swirls, and sheens like mahogany, flame, pumpkin, rainbow, silver sheen, lizard skin, snowflakes, midnight lace, golden glow, etc. Obsidian is usually pure or jet black, dark gray, or dark brownish in color. It will further broaden your understanding of this volcanic glass. Let us explore colors, banding or layering, and metastable state. Also, it resembles the black, green-gray, or brown gravel-sized tektites from meteorite impacts. These properties should help you identify real and fake ones.Īlso, 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. However, it may be translucent or rarely transparent. It is mostly opaque with translucent edges and may show flow banding unique colors, some wavy or swirly. 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. Usually, obsidian is jet black, dark gray, or brownish, but it can be other colors. However, mafic magmas hardly form volcanic glasses.Ĭolor, fracture, banding, luster, Mohs hardness scale, and general appearance should help you identify obsidian. Also, trachyte is a mafic (usually basaltic) volcanic glass. For instance, you can have dacite-obsidian.īesides obsidian, other natural glass examples are pumice, Pele’s hair, Pele’s tears, perlite, and pitchstone. If it forms from non-rhyolitic magma, it will bear a prefix to indicate which magma type it is.

However, it can form from other acidic, siliceous, or silica-rich magma or lava. The quick cooling and high viscosity prevent crystal formation. Usually, obsidian is a rhyolite glass formed from rapid quenching viscous, silica-rich lava or magma. However, it is often considered a volcanic or an extrusive igneous rock. Obsidian is a hard, brittle, natural volcanic glass or mineraloid with a conchoidal fracture. Some may show chatoyancy if they have parallel inclusion or the Schiller effect. Optical properties: It is transparent, translucent, or opaque except for the edges.Resembles: It resembles tektite, onyx (a mineral), chalcedony, jet, and schorl.Equivalent: Chemically equivalent to crystalline rocks like rhyolite or granite.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.Tiny crystals of magnetite inclusions, a magnetic mineral, make it weakly magnetic.

Cooling history: Fast on the Earth’s surface allows minimal mineral formation.

Fracture: Conchoidal (since it is a glass without minerals) like quartz, chert, or flint.

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.

Origin: Extrusive or volcanic but can be intrusive when on sills or dike edges.

It feels like a _, CC BY-SA 3.0, via Wikimedia Commons. Here is an obsidian rock from Lipari in Italy: Photo credit: Ji-Elle It feels nice and warm. We will discuss its appearance and colors, composition, formation, uses, and where it is found or occurs. Also, you can use it in landscaping or making ornamental items. It can potentially make the sharpest surgical blades. Today, gemstone quality makes beads, necklaces, anklets, pendants, rings, earrings, etc. This volcanic glass made prehistoric weapons like arrowheads, swords, daggers, and tools like mirrors, blades, knives, etc. 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. Both Pliny the Elder (Gaius Plinius Secundus) and Pliny the Younger attest to this. The term ‘obsidian’ may have come from Obsidius, a Roman explorer who discovered it in Ethiopia. The rapid cooling and high viscosity results in an amorphous solid or non-crystalline structure. This rock is mostly extrusive (with some intrusive) and forms from the rapid cooling of highly viscous, often rhyolitic, or granitic magma. It has 69-77% silica and is relatively high in alkalis (>7%) and low in iron and magnesium oxides. Obsidian is a felsic, naturally occurring volcanic glass.

Natural obsidian properties