Obsidian is hard,brittle, andamorphous; it thereforefractures with sharp edges. In the past, it was used to manufacture cutting and piercing tools, and it has been used experimentally as surgicalscalpel blades.[8]
Origin and properties
TheNatural History by the Roman writerPliny the Elder includes a few sentences about a volcanic glass called obsidian (lapis obsidianus), discovered in Ethiopia by Obsidius, a Roman explorer.[9][10][11][12]
Obsidian is formed from quickly cooledlava.[13][14][15] Extrusive formation of obsidian may occur whenfelsic lava cools rapidly at the edges of a felsic lava flow or volcanic dome, or when lava cools during sudden contact with water or air. Intrusive formation of obsidian may occur when felsic lava cools along the edges of adike.[16][17]
Tektites were once thought by many to be obsidian produced bylunar volcanic eruptions,[18] though few scientists now adhere to thishypothesis.[19]
Obsidian is mineral-like, but not a true mineral because, as a glass, it is notcrystalline; in addition, its composition is too variable to be classified as a mineral. It is sometimes classified as amineraloid.[20] Though obsidian is usually dark in color, similar tomafic rocks such asbasalt, the composition of obsidian is extremely felsic. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% by weight or more; the remainder consists of variable amounts of other oxides, mostly oxides of aluminium, iron, potassium, sodium and calcium.[21][22] Crystalline rocks with a similar composition includegranite andrhyolite. Because obsidian ismetastable at the Earth's surface (over time the glassdevitrifies, becoming fine-grained mineral crystals), obsidian older thanMiocene in age is rare. Exceptionally old obsidians include aCretaceous welded tuff and a partially devitrifiedOrdovicianperlite.[23] This transformation of obsidian is accelerated by the presence of water. Although newly formed obsidian has a low water content, typically less than 1% water by weight,[24] it becomes progressivelyhydrated when exposed togroundwater, formingperlite.
Polished snowflake obsidian, formed through the inclusion ofcristobalite crystals
Pure obsidian is usually dark in appearance, though the color varies depending on the impurities present. Iron and othertransition elements may give the obsidian a dark brown to black color. Most black obsidians containnanoinclusions ofmagnetite, aniron oxide.[25] Very few samples of obsidian are nearly colorless. In some stones, theinclusion of small, white, radially clustered crystals (spherulites) of the mineralcristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). Obsidian may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden sheen (sheen obsidian). Aniridescent,rainbow-like sheen (fire obsidian) is caused by inclusions ofmagnetitenanoparticles creatingthin-film interference.[26] Colorful, striped obsidian (rainbow obsidian) from Mexico contains orientednanorods ofhedenbergite, which cause the rainbow striping effects bythin-film interference.[25]
The first known archaeological evidence of usage was inKariandusi (Kenya) and other sites of theAcheulian age (beginning 1.5 million years BP) dated 700,000 BC, although only very few objects have been found at these sites relative to the Neolithic.[37][38][39][40][41] Manufacture of obsidian bladelets atLipari had reached a high level of sophistication by the late Neolithic, and was traded as far as Sicily, the southern Po river valley, and Croatia.[42] Obsidian bladelets were used in ritualcircumcisions and cutting of umbilical cords of newborns.[43] Anatolian sources of obsidian are known to have been the material used in the Levant and modern-day Iraqi Kurdistan from a time beginning sometime about 12,500 BC.[44] Obsidian artifacts are common atTell Brak, one of the earliest Mesopotamian urban centers, dating to the late fifth millennium BC.[45] Obsidian was valued inStone Age cultures because, likeflint, it could be fractured to produce sharp blades or arrowheads in a process calledknapping. Like all glass and some other naturally occurring rocks, obsidian breaks with a characteristicconchoidal fracture. It was also polished to create earlymirrors. Modernarchaeologists have developed arelative dating system,obsidian hydration dating, to calculate the age of obsidianartifacts.
Obsidian artifacts first appeared in the European continent in Central Europe in theMiddle Paleolithic and had become common by theUpper Paleolithic, although there are exceptions to this. Obsidian played an important role in thetransmission of Neolithic knowledge and experiences. The material was mainly used for production ofchipped tools which were very sharp due to its nature. Artifacts made of obsidian can be found in many Neolithic cultures across Europe. The source of obsidian for cultures inhabiting the territory of and around Greece was the island ofMilos; theStarčevo–Körös–Criș culture obtained obsidian from sources in Hungary and Slovakia, while theCardium-Impresso cultural complex acquired obsidian from the island outcrops of the centralMediterranean. Through trade, these artifacts ended up in lands thousands of kilometers away from the original source; this indicates that they were a highly valued commodity.[46] X-ray fluorescence techniques have also allowed obsidian in Greece to be identified as coming fromMilos,Nisyros orGyali, islands in theAegean Sea[citation needed].John Dee had a mirror, made of obsidian, which was brought from Mexico to Europe between 1527 and 1530 after Hernando Cortés's conquest of the region.[47]
In theUbaid in the5th millennium BC, blades were manufactured from obsidian extracted from outcrops located in modern-dayTurkey.[48]Ancient Egyptians used obsidian imported from the eastern Mediterranean and southernRed Sea regions. Obsidianscalpels older than 2100 BC have been found in a Bronze Age settlement in Turkey.[49] In the eastern Mediterranean area the material was used to make tools, mirrors and decorative objects.[50]
Obsidian has also been found inGilat, a site in the westernNegev in Israel. Eight obsidian artifacts dating to theChalcolithic Age found at this site were traced to obsidian sources inAnatolia.Neutron activation analysis (NAA) on the obsidian found at this site helped to reveal trade routes and exchange networks previously unknown.[53]
Obsidian worked into plates and other wares by Victor Lopez Pelcastre of Nopalillo, Epazoyucan, Hidalgo. On display at theMuseo de Arte Popular, Mexico City.
Indigenous people traded obsidian throughout the Americas. Eachvolcano and in some cases each volcanic eruption produces a distinguishable type of obsidian allowing archaeologists to use methods such as non-destructive energy dispersive X-ray fluorescence to select minor element compositions from both the artifact and geological sample to trace the origins of a particular artifact.[54] Obsidian cores and blades were traded far inland from the coast.[55] A particularly distant examples of traded obsidian are pieces from theYellowstone Region found atHopewell sites, such asHopewell Culture National Historical Park, Ohio, over 1500 miles away.[56]
This Obsidian scraper found at Spiro Mounds, Oklahoma, was sourced from Pachuca, Mexico; an example of vast Pre-Columbian trade networks
Obsidian tools found inMission Santa Clara has shown the existence of exchange networks between various tribes inCalifornia.Lithic analysis helps to understand pre-Hispanic groups inMesoamerica. A careful analysis of obsidian in a culture or place can be of considerable use to reconstruct commerce, production, and distribution, and thereby understand economic, social and political aspects of a civilization. For example, the coastalChumash sites in California indicate considerable trade with the distant site ofCasa Diablo Hot Springs in theSierra Nevada[57]. Obsidian in California comes from 5 major locations all around the state, and when Mission Santa Clara was built, the tribes took their obsidian tools with them and from the analysis of the obsidian tools it showed that all 5 major location of obsidian were present.[58]. While in Mesoamerica, at the Maya city ofYaxchilán, even warfare implications have been studied linked with obsidian use and its debris.[59] GreenPachuca obsidian was highly prized: it has been argued Teotihuacan monopolized the Pachuca deposit to control and influence Obsidian trade in Central Mexico during the Classic Period[60], and theMexica ofTenochtitlan (the capital of the Aztec Empire), favored Pachuca obsidian for ritual deposits after rising to power.[61] A scraper made from Pachuca obsidian has even been found atSpiro Mounds inOklahoma.[62]
Raw obsidian and obsidian blades from the Mayan site of Takalik Abaj
Pre-Columbian Mesoamericans'use of obsidian was extensive and sophisticated; including carved and worked obsidian for tools and decorative objects. Mesoamericans made use of a variety of weapons using obsidian, such asmacuahuitl, a type of sword with obsidian blades lining a wooden shaft, or thetepoztopilli, a polearm with a leaf or spade shaped wooden head lined with blades in a similar manner. Spanish sources describe these weapons as being able kill and inflicting terrible injuries.[63]
Obsidian mirrors were used by some Aztec priests to conjure visions and make prophecies. They were connected withTezcatlipoca, god of obsidian and sorcery, whose name can be translated from the Nahuatl language as 'Smoking Mirror'.[47]
In Chile obsidian tools fromChaitén Volcano have been found as far away as inChan-Chan 400 km (250 mi) north of the volcano, and also in sites 400 km south of it.[64][65]
Oceania
TheLapita culture, active across a large area of the Pacific Ocean around 1000 BC, made widespread use of obsidian tools and engaged in long distance obsidian trading. The complexity of the production technique for these tools, and the care taken in their storage, may indicate that beyond their practical use they were associated with prestige or high status.[66]
Obsidian was also used onRapa Nui (Easter Island) for edged tools such asMataia and the pupils of the eyes of theirMoai (statues), which were encircled by rings of bird bone.[67] Obsidian was used to inscribe theRongorongo glyphs.
Current use
Obsidian can be used to make extremely sharp knives, and obsidian blades are a type ofglass knife made using naturally occurring obsidian instead of manufactured glass. Obsidian is used by some surgeons forscalpel blades, although this is not approved by theUS Food and Drug Administration (FDA) for use on humans.[68] Well-crafted obsidian blades, like any glass knife, can have a cutting edge many times sharper than high-quality steel surgical scalpels: the cutting edge of the blade is only about three nanometers thick.[69] All metal knives have a jagged, irregular blade when viewed under a strong enoughmicroscope; however, obsidian blades are still smooth, even when examined under anelectron microscope.[70] One study found that obsidian incisions produced fewerinflammatory cells and lessgranulation tissue in a group of rats after seven days but the differences disappeared after twenty-one days.[71]Don Crabtree has produced surgical obsidian blades and written articles on the subject.[69] Obsidian scalpels may be purchased for surgical use onresearch animals.[72]
The major disadvantage of obsidian blades is their brittleness compared to those made of metal,[73] thus limiting the surgical applications for obsidian blades to a variety of specialized uses where this is not a concern.[69]
Pig carved in snowflake obsidian, 10 centimeters (4 in) long. The markings arespherulites.
Obsidian is also used for ornamental purposes and as agemstone.[74] It presents a different appearance depending on how it is cut: in one direction it is jet black, while in another it is glistening gray. "Apache tears" are small rounded obsidian nuggets often embedded within a grayish-white perlitematrix.
Plinths foraudio turntables have been made of obsidian since the 1970s, such as the grayish-black SH-10B3 plinth byTechnics.
See also
Apache tears – Popular term for pebbles of obsidian
Helenite – Artificial glass made from volcanic ash
Yaxchilan Lintel 24 – Ancient Maya limestone carving from Yaxchilan in modern Chiapas, Mexico – Ancient carving showing a Maya bloodlet ritual involving a rope with obsidian shards.
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