Home Igneous Rocks Extrusive Igneous Rocks Obsidian



Obsidian is an extrusive igneous rock. It is occurring as a natural glass formed by the rapid cooling of viscous lava from volcanoes. Result of this formed, glass occupies more than 80 % and that have seeming conchoidal fracture and vitreous luster. It usually colours dark, black, grey-black to grey. Also It occur red, brown-green, green, yellow and rare transparent colourless. Colour is controlled factor by trace element content and the character of inclusion. Hand-specimen rock is common combination of two colours. Those are combination of black and brown colours.

Obsidian most frequently originates by a rapid coooling of a felsic viscous lava. Diffusion in the very viscous acid and felsic lavas with highly polymerized SiO2 tetrahedrons is slow, thus inhibiting the nucleation of crystals and supporting the origin of the glassy structure. Obsidian is fragile, tough and it disintegrates to very sharp slivers.

Name origin: The name obsidian is very old. It is ascribed to Theophrastus who used it for the first time in 320 BC.The translation into English of Natural History written by Pliny the Elder of Rome shows a few sentences on the subject of a volcanic glass called obsidian (lapis obsidianus), discovered in Ethiopia by Obsidius, a Roman explorer

.among the various forms of glass we may reckon Obsidian glass, a substance very similar to the stone found by Obsidius in Ethiopia.

— Pliny the Elder, Naturalis Historia 36,67 (AD 77)

Colour: Black

Structure: Massive

Texture: Glassy (vitreous, hyaline).

Alterations: Glass is neither altered nor devitrified.

Major minerals: Amorphous volcanic glass that exhibits the structural ordering similar to crystals only in very short distances. For this reason, the obsidian is sometimes regarded as a mineraloid.

Classification of Obsidian

Volcanic rocks containing more than 80 vol. % of glass do not have their own classification, but they can be characterized using the TAS diagram in terms of their chemical composition. However, individual types of volcanic glasses also differ in terms of their structure and texture.

Air bubbles that are imparted in the course of magma flow, are aligned in a regular pattern in some obsidian types. Sheen obsidian has golden effects, while rainbow obsidian possesses characteristic rainbow luster. Another common black type is snowflake obsidian, having inclusions of small white crystals.

Chemical Composition of Obsidian

Chemical composition of obsidian is close to these rock rhyolite and granite. It is dominant SiO2, MgO and Fe2O3 is most commont component. These affact the colour of obsidian. Al2O3 content is lower than common rhyolite and granite.As a rule, granite and rhyolite are peraluminous, with corundum in their norms, whereas the Lipari obsidian is metaluminous, devoid of the normative corundum. Obsidian is unstable in atmospheric conditions and it temporally crystallizes to very finely grained crystal aggregate. Weathering is accelerated in the presence of water. Obsidians from various localities in the World differ partly in terms of their trace element contents.

Rarely founded is a composition similar to basalt and gabbro.These name of galasy are  tachylyte

Formation of the Obsidian

Obsidian is produced from volcanoes when felsic lava cools and freezes without sufficient time for crystal growth. It is commonly found in areas where the cooling of lava was rapid—such as at the margins of the lava flows and in places where the lava entered a lake or sea. As silicate-rich magma extrudes from inside the earth’s crust, it falls over the water bodies or other surfaces that are in contact with water. As a result, the lava gets cooled and consolidated instantly without any crystallization. Hence, obsidian cannot be classified under any crystal system.This glassy rock is metastable—over time, it becomes fine-grained, mineral crystals.

Where is Obsidian Located

Obsidian is found in many locations worldwide. It is confined to areas of geologically recent volcanic activity. Obsidian older than a few million years is rare because the glassy rock is rapidly destroyed or altered by weathering, heat, or other processes.

Significant deposits of obsidian are found in Argentina, Canada, Chile, Ecuador, Greece, Guatemala, Hungary, Iceland, Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, Peru, Russia, United States, and many other locations.

Characteristics and Properties of Rock

Petrographic characteristics unaltered compact obsidian with black colour, pronounced conchoidal fracture and sharp edges. It does not contain any macroscopically discernible inclusions of gases, liquids or solids, such as minerals or rock fragments. The obsidian exhibits a pronounced vitreous lustre. It does not scratch glass, thus corresponding to the hardness of 5 or 6 in the Mohs scale. It is translucent in thin cleavage fragments and along sharp margins.

Uses of The Obsidian

Due to their attractive glassy texture, it is used as type of semiprecious stones. Also, it is kept as decorative specimens, especially after designs are carved over its surface. It are also used in the manufacturing of surgical blades. Unlike steel and metallic blades, the cutting edge of an obsidian blade is even throughout its length, thus giving smooth cuts.

The first use of its by people probably occurred when a sharp piece of obsidian was used as a cutting tool. People then discovered how to skillfully break the obsidian to produce cutting tools in a variety of shapes. It was used to make knives, arrowheads, spear points, scrapers, and many other weapons and tools.

Once these discoveries were made, It quickly became the raw material of preference for producing almost any sharp object. The easy-to-recognize rock became one of the first targets of organized “mining.” It is probably a safe bet that all natural outcrops that are known today were discovered and utilized by ancient people.

Freshly broken pieces of obsidian have a very high luster. Ancient people noticed that they could see a reflection in obsidian and used it as a mirror. Later, pieces of obsidian were ground flat and highly polished to improve their reflective abilities.

It’s hardness of 5.5 makes it relatively easy to carve. Artists have used to make masks, small sculptures, and figurines for thousands of years.

Facts About The Rock

  • Different varieties of obsidian rock get their names from their appearance.
  • The appearance of obsidian is based on the chemical composition of the magma that cooled to make the rock.
  • It is also known as “nature’s glass” because of its glassy appearance.
  • When It breaks, the fractures are very sharp which is why it was used as tools in the Stone Age.
  • Seventy percent of obsidian rock is made of silica.
  • It can only form near active volcanoes.
  • Sometimes It is erupted directly from a volcano.
  • The best is formed underground near a volcanic vent because those rocks will have limited impurities.
  • It flows are so slow that often other obsidian flows will occur on top of each other causing a streaked appearance in the rock.
  • Tiny gas bubbles that trap water are a part rock.
  • Although most rocks that form in the Earth’s crust are very old, obsidian is rarely older than 20 million years old which is young for a rock.
  • It goes through a process called devitrification whereby it turns from glass to a rock.
  • It is commonly used during surgical procedures because it is often sharper than traditional surgical tools.
  • It is used as a gemstone in jewelry.


Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.

Wikipedia contributors. (2019, April 8). Obsidian. In Wikipedia, The Free Encyclopedia. Retrieved 15:17, April 10, 2019, from https://en.wikipedia.org/w/index.php?title=Obsidian&oldid=891494770

Obsidian. (2018, December 17). New World Encyclopedia, . Retrieved 15:17, April 10, 2019 from http://www.newworldencyclopedia.org/p/index.php?title=Obsidian&oldid=1016455.

Atlas-hornin.sk. (2019). Atlas of magmatic rocks. [online] Available at: http://www.atlas-hornin.sk/en/home [Accessed 13 Mar. 2019].


Cite this article as: Mahmut Mat, "Obsidian", in Geology Science, [online] Accessed 21st August 2019, Available at: http://geologyscience.com/minerals/obsidian/