The Chemical Composition and Physical Properties of Obsidian

Obsidian is volcanic glass and igneous rock formed from silicon-rich molten magma. Although the most common or well-known obsidian is black, the volcanic glass does come in different colors including “mahogany, red, flame, midnight lace, jet black, pumpkin, brown, rainbow, gold sheen, silver sheen, green, lizard skin, snowflake and more,” (Miller 1). The color variations are due to the iron, magnesium, and other “impurities,” (“Obsidian: Definition, Properties & Uses”). As a glass, more than 70% of obsidian is comprised of silicon dioxide (silica), but unlike quartz, obsidian does not crystallize. Obsidian is formed from magma that cools relatively close to the surface, in the absence of water but in the presence of oxygen, resulting in the creation of polymer chains (“Obsidian: Definition, Properties & Uses”). Unlike pumice and other volcanic rocks, though, obsidian has no gas bubbles.
Obsidian is chemically similar to granite, which can also form from magma. However, granite forms deeper beneath the surface of the earth, cooling slowly to allow the mineral crystals to form (Miller). Also unlike granite or its relative quartz, obsidian has no mineral crystals and therefore has its characteristic glassiness (Miller). The lack of mineral crystallization in obsidian is due both to its surface-level cooling but also to the polymerization process (“Obsidian: Definition, Properties & Uses”). Because of the lack of mineral crystallization in obsidian, it has often been classified as a “mineralioid,” rather than being deemed a true mineral (“Obsidian: Definition, Properties & Uses”).
Obsidian’s chemical composition and its physical properties have made it one of the most important mineral resources in material culture around the world. Available in volcanic regions, the most known areas of human extraction and use of obsidian include the Near East/Mediterranean, the South Pacific, and the Americas. Based on the archaeological record, “obsidian was not necessarily the earliest object of trade, but it certainly seems to be the first for which material evidence remains,” (Renfrew, Dixon and Cann 30). As one of the earliest known traded items, obsidian in the archaeological record lends great insight into the ways early cultures used the mineral in tools.
Obsidian has been used in tools since the Neolithic era, when it was used for trepanation and other “emergency surgery” situations (Shadbolt 1). The reasons why obsidian made such an apt surgical instrument include its physical properties. Although obsidian is softer than quartz on the mineral hardness scale, obsidian fractures in ways that create very sharp edges—like all glass. The way obsidian fractures is called conchoidal,” (Miller; “Obsidian: Definition, Properties & Uses”). In fact, obsidian “can produce cutting edges many times finer than even the best steel scalpels,” even rivaling diamond in the “fineness of its edge,” (Shadbolt 1). With a naturally honed edge, an obsidian blade cuts cleanly and allows the wound to heal fast and without scarring (Shadbolt).
Because obsidian is often found in sites far from its volcanic origin, studying its presence in archaeological sites around the world also offers insight into Neolithic global trade routes during the dawn of civilization. Obsidian trading reveals patterns of human interaction that can also demonstrate the broader exchange of other goods and services, explaining things like how different plants and animals spread throughout large geographic areas. Studying the obsidian record can be “the most promising approach towards understanding the extent to which the different Early Neolithic cultural and ecological regions were in contact,” (Renfrew, Dixon and Cann 30). This is especially true for North America, the Near East, and the South Pacific, where obsidian is in relative abundance. Obsidian has been used in a variety of contexts including scraping and tanning hides, as arrowheads and other weaponry, and for tattoo art (Daley).




Works Cited

Daley, Jason. “Did Ancient Pacific Islanders Use Obsidian to Make Their Tattoos? Smithsonian. July 13, 2016. Retrieved online: http://www.smithsonianmag.com/smart-news/did-ancient-pacific-islanders-use-obsidian-make-their-tattoos-180959791/#GGohZYFgxtwAcwH4.99
Miller, Jim. “Obsidian is Hot Stuff.” Retrieved online: http://volcano.oregonstate.edu/book/export/html/205
“Obsidian: Definition, Properties & Uses” Geology. Retrieved online: https://www.geologyin.com/2015/08/obsidian-definition-properties-uses.html
Renfrew, Colin, Dixon, J.E. and Cann, J.R. “Obsidian and Early Cultural Contact in the Near East.” Proceedings of the Prehistoric Society, Vol. 32, Dec 1966, pp. 30-72.
Shadbolt, Peter. “How Stone Age blades are still cutting it in modern surgery.” CNN. April 2, 2015. Retrieved online: http://www.cnn.com/2015/04/02/health/surgery-scalpels-obsidian/index.html