Obsidian

Obsidian is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows. These flows have a high content of silica, giving them a high viscosity. The high viscosity inhibits the diffusion of atoms through the lava, which inhibits the first step (nucleation) in the formation of mineral crystals. Together with rapid cooling, this results in a natural glass forming from the lava.

Obsidian is hard, brittle, and amorphous; it therefore fractures with sharp edges. In the past, it was used to manufacture cutting and piercing tools, and it has been used experimentally as surgical scalpel blades.

Obsidian

General

Category Volcanic glass

Identification

Color usually black; sometimes green or brown; rarely yellow, orange, red or blue

Fracture Conchoidal

Mohs scale hardness 5–6

Luster Vitreous

Specific gravity c. 2.4

Optical properties Translucent

Melting point 700–1,050 °C (1,292–1,922 °F)

Other characteristics Texture: Smooth; glassy

Origin and properties

The Natural History by the Roman writer Pliny the Elder includes a few sentences about a volcanic glass called obsidian (lapis obsidianus), discovered in Ethiopia by Obsidius, a Roman explorer.

Obsidian is formed from quickly cooled lava. Extrusive formation of obsidian may occur when felsic 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 a dike.

Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis.

Obsidian is mineral-like, but not a true mineral because, as a glass, it is not crystalline; in addition, its composition is too variable to be classified as a mineral. It is sometimes classified as a mineraloid. Though obsidian is usually dark in color, similar to mafic rocks such as basalt, 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. Crystalline rocks with a similar composition include granite and rhyolite. 

Because obsidian is metastable at the Earth's surface (over time the glass devitrifies, becoming fine-grained mineral crystals), obsidian older than Miocene in age is rare. Exceptionally old obsidians include a Cretaceous welded tuff and a partially devitrified Ordovician perlite. 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, it becomes progressively hydrated when exposed to groundwater, forming perlite.

Pure obsidian is usually dark in appearance, though the color varies depending on the impurities present. Iron and other transition elements may give the obsidian a dark brown to black color. Most black obsidians contain nanoinclusions of magnetite, an iron oxide. Very few samples of obsidian are nearly colorless. In some stones, the inclusion of small, white, radially clustered crystals (spherulites) of the mineral cristobalite 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). An iridescent, rainbow-like sheen (fire obsidian) is caused by inclusions of magnetite nanoparticles creating thin-film interference. Colorful, striped obsidian (rainbow obsidian) from Mexico contains oriented nanorods of hedenbergite, which cause the rainbow striping effects by thin-film interference.

Physical characteristics

Obsidian is opaque to translucent and has a glassy texture and luster. It is most often gray or black, but sometimes obsidian reflects light, depending on its internal planes, so that green, purple and silver reflections appear, this phenomenon is called "obsidian rainbow". Its fracture is very clearly conchoidal, its hardness on the Mohs scale being 5 to 5.5 (like glass).

Obsidian glass can recrystallize, resulting in cristobalite spherulites or, sometimes, "snowflake" obsidian with recrystallization orbicules. Due to the rapid cooling, regular crystal structures do not form. The material of which obsidian is composed thus has a chaotic, amorphous structure. Since the crystalline structure is missing, obsidian, as a rock glass, is by definition a supercooled liquid.

Like all glasses, obsidian is metastable and tends to devitrify and crystallize over geological time. This process also allows the formation of spherulites, mineral aggregates of radially arranged crystals (eye or snowflake obsidian). One rock derived from obsidian is perlite, which is used in construction. Volcanic glasses from the Paleozoic and Precambrian eras are unknown—with the exception of pitchstone—because they are now completely devitrified.

Most obsidians have a silica content of at least 70% and belong to the rhyolite family – rhyolites are the volcanic equivalents of granites. Rarer are trachitic, andesitic, and phonolitic obsidians with lower silica contents.

The color varies greatly depending on the presence of various impurities and their oxidation states. Despite its usually high silica content (for comparison, granites are usually light-colored rocks), obsidian is usually dark green to black, occasionally brown and reddish. This is due to the finely distributed hematite or magnetite minerals in the rock.

Depending on the deposit, however, crystals may be embedded in the glassy (hyaline) structure in greater or lesser quantities. The often developed flow texture manifests itself in a streaky appearance (eutaxitic structure).

Varieties

Snowflake obsidians contain inclusions of radially grown structures, up to 1 cm in size, called spherulites. These minerals, usually feldspars or cristobalite (a high-temperature modification of quartz), grew spherically from a crystal nucleus into the surrounding melt until cooling stopped this process.

Small lumps of obsidian, rounded by erosion, are called Apache tears (also smoky obsidian). Folk belief holds that an Indian died at the site of an Apache tear.

Occurrence

Obsidian is found near volcanoes in locations which have undergone rhyolitic eruptions. It can be found in Argentina, Armenia, Azerbaijan, Australia, Canada, Chile, Georgia, Ecuador, El Salvador, Greece, Guatemala, Hungary, Iceland, Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, Papua New Guinea, Peru, Russia, Scotland, the Canary Islands, Turkey and the United States. Obsidian flows which are so large that they can be hiked on are found within the calderas of Newberry Volcano (Big Obsidian Flow, 700 acres) and Medicine Lake Volcano in the Cascade Range of western North America, and at Inyo Craters east of the Sierra Nevada in California. Yellowstone National Park has a mountainside containing obsidian located between Mammoth Hot Springs and the Norris Geyser Basin, and deposits can be found in many other western U.S. states including Arizona, Colorado, New Mexico, Texas, Utah, and Washington, Oregon and Idaho.

There are only four major deposit areas in the central Mediterranean: Lipari, Pantelleria, Palmarola and Monte Arci (Sardinia).

Ancient sources in the Aegean were Milos and Gyali.

Acıgöl town and the Göllü Dağ volcano were the most important sources in central Anatolia, one of the more important source areas in the prehistoric Near East.

Prehistoric use

The first known archaeological evidence of usage was in Kariandusi (Kenya) and other sites of the Acheulian 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. Manufacture of obsidian bladelets at Lipari 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. Obsidian bladelets were used in ritual circumcisions and cutting of umbilical cords of newborns. 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. 

Obsidian artifacts are common at Tell Brak, one of the earliest Mesopotamian urban centers, dating to the late fifth millennium BC. Obsidian was valued in Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads in a process called knapping. Like all glass and some other naturally occurring rocks, obsidian breaks with a characteristic conchoidal fracture. It was also polished to create early mirrors. Modern archaeologists have developed a relative dating system, obsidian hydration dating, to calculate the age of obsidian artifacts.

Obsidian was used for the manufacture of cutting edges for weapons and tools during prehistory, notably in pre-Columbian America (see in more detail the article on the use of obsidian in Mesoamerica). Rolled pebbles are used in the manufacture of jewelry and the hardness of this mineral and its relative ease of cutting allow it to be made into blades. In South America on the Pacific coast obsidian was the subject of long-distance trade, notably for certain shells (spondyls and conchs). In Mesoamerica in the first millennium AD obsidian deposits made the wealth of Teotihuacan, whose production is found as far back as Altun Ha.

There are also numerous traces of obsidian use in southern Europe during the Neolithic period, where a form of trade and transport of the stone was established from the deposits of the volcanoes of present-day Italy. Obsidian tools have been found during archaeological excavations, particularly in the south of France. From the Neolithic period in the western Mediterranean, 8,000 years ago, exchange networks were established between the different agropastoral communities, networks which remained active for four millennia.

A recent study of prehistoric obsidian sheds light on how some of these long-distance exchanges between the islands and the Mediterranean shores were organized: this trade was partly in the hands of specialized craftsmen who traveled by sea and distributed the product of their crafts to the communities they visited. Archaeological sites attest to the shaping of cores around the deposits (mainly Lipari and Monte Arci in Sardinia), then to the detachment of blades and bladelets in the "consumer" villages. In Sardinia, obsidian was an important resource, exploited since the first phases of the Early Neolithic (6000-5000 BCE). Used to make weapons, tools and other lithic objects, obsidian is much rarer than common flint and has better characteristics (brightness, easy to shape); It was therefore highly sought after in Antiquity and around some deposits, there flourished an intense activity of extraction, transformation and exchange even over long distances.

In East Africa, on the shores of the Gulf of Winam (Lake Victoria), obsidian tools have been found in archaeological sites dating from between the 6th millennium BC and the 3rd millennium BC; chemical analyses have shown that the obsidian used came from deposits located more than 150 km away, on the shores of Lake Naivasha or Lake Bogoria. Some tools have been found 400 km from their extraction site. These exchanges probably had a social and symbolic purpose rather than a utilitarian one.

In prehistoric Japan, obsidian was mined in various places and was traded over several hundred kilometers, or even more in the case of maritime trade; obsidian objects from Kyushu have been found in the Ryukyu Islands and others from Hokkaido have been unearthed north of Sakhalin.

The discovery of obsidian on an archaeological site is a valuable source of information, as it makes it possible to date it using the hydration method. In addition, each deposit has its own chemical composition, which makes it possible to deduce the place of extraction of the obsidian constituting an artifact found during an excavation.

Europe

Obsidian artifacts first appeared in the European continent in Central Europe in the Middle Paleolithic and had become common by the Upper Paleolithic, although there are exceptions to this. Obsidian played an important role in the transmission of Neolithic knowledge and experiences. The material was mainly used for production of chipped 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 of Milos; the Starčevo–Körös–Criș culture obtained obsidian from sources in Hungary and Slovakia, while the Cardium-Impresso cultural complex acquired obsidian from the island outcrops of the central Mediterranean. 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. 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.

Middle East and Asia

In the Ubaid in the 5th millennium BC, blades were manufactured from obsidian extracted from outcrops located in modern-day Turkey. Ancient Egyptians used obsidian imported from the eastern Mediterranean and southern Red Sea regions. Obsidian scalpels older than 2100 BC have been found in a Bronze Age settlement in Turkey. In the eastern Mediterranean area the material was used to make tools, mirrors and decorative objects.

The use of obsidian tools was present in Japan near areas of volcanic activity. Obsidian was mined during the Jōmon period.

Obsidian has also been found in Gilat, a site in the western Negev in Israel. Eight obsidian artifacts dating to the Chalcolithic Age found at this site were traced to obsidian sources in Anatolia. Neutron activation analysis (NAA) on the obsidian found at this site helped to reveal trade routes and exchange networks previously unknown.

Americas

Lithic analysis helps to understand pre-Hispanic groups in Mesoamerica. 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. This is the case in Yaxchilán, a Maya city where even warfare implications have been studied linked with obsidian use and its debris. Another example is the archeological recovery at coastal Chumash sites in California, indicating considerable trade with the distant site of Casa Diablo Hot Springs in the Sierra Nevada.

Pre-Columbian Mesoamericans' use of obsidian was extensive and sophisticated; including carved and worked obsidian for tools and decorative objects. Mesoamericans also made a type of sword with obsidian blades mounted in a wooden body. Called a macuahuitl, the weapon could inflict terrible injuries, combining the sharp cutting edge of an obsidian blade with the ragged cut of a serrated weapon. The polearm version of this weapon was called tepoztopilli.

Obsidian mirrors were used by some Aztec priests to conjure visions and make prophecies. They were connected with Tezcatlipoca, god of obsidian and sorcery, whose name can be translated from the Nahuatl language as 'Smoking Mirror'.

Indigenous people traded obsidian throughout the Americas. Each volcano 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. Similar tracing techniques have also allowed obsidian in Greece to be identified as coming from Milos, Nisyros or Gyali, islands in the Aegean Sea. Obsidian cores and blades were traded great distances inland from the coast.

In Chile obsidian tools from Chaitén Volcano have been found as far away as in Chan-Chan 400 km (250 mi) north of the volcano, and also in sites 400 km south of it.

Oceania

The Lapita 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.

Obsidian was also used on Rapa Nui (Easter Island) for edged tools such as Mataia and the pupils of the eyes of their Moai (statues), which were encircled by rings of bird bone. Obsidian was used to inscribe the Rongorongo glyphs.

Current use

Obsidian can be used to make extremely sharp knives, and obsidian blades are a type of glass knife made using naturally occurring obsidian instead of manufactured glass. Obsidian is used by some surgeons for scalpel blades, although this is not approved by the US Food and Drug Administration (FDA) for use on humans. 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. All metal knives have a jagged, irregular blade when viewed under a strong enough microscope; however, obsidian blades are still smooth, even when examined under an electron microscope. One study found that obsidian incisions produced fewer inflammatory cells and less granulation tissue in a group of rats after seven days but the differences disappeared after twenty-one days. Don Crabtree has produced surgical obsidian blades and written articles on the subject. Obsidian scalpels may be purchased for surgical use on research animals.

The major disadvantage of obsidian blades is their brittleness compared to those made of metal, thus limiting the surgical applications for obsidian blades to a variety of specialized uses where this is not a concern.

Obsidian is also used for ornamental purposes and as a gemstone. 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 perlite matrix.

Plinths for audio turntables have been made of obsidian since the 1970s, such as the grayish-black SH-10B3 plinth by Technics.

Hydration dating of obsidian

Obsidian has been valuable as a research tool that can be used to discover the age of ancient civilizations. The rate of hydration of obsidian with hydrocarbons in years has been established for several locations around the world. When natural crystal fractures during the manufacture of artifacts such as projectile points, newly exposed surfaces are provided in the chipping flakes and finished specimens. These surfaces absorb ambient water to form hydrated surface layers that differ in density and refractive index from the rest of the obsidian. The chemical composition of the obsidian and temperature are two factors that control the hydration rate. The thickness of the hydrated layer depends on time, temperature, and the chemical composition of the material. This section of each specimen is evaluated based on the hydration layer, and the layer thickness is measured in the magnification used in transmitted light to calculate local hydration rates for obsidians of different chemical compositions found in different climatic regions. Correlations are made between the large number of obsidian measurements and the conventional context of the associated carbon-14 age in each area. Since the hydration rate depends on the temperature of the locality, a given rate is applicable only to the climate of the area where the measurement was made. 

Crafts, trade, art and folklore

There are aesthetic or commercial names for varieties, notably according to the curative virtues that popular culture grants them based on their appearance: the "rainbow" (also called in France "celestial eye"), the "mahogany" (" mahogany "), the "snowflake" ("speckled"), the black, the golden, the silver, the " mentogochol " (deformation of the Mexican name Manta Huichol), the " spider web " ("spider web") or the " mezclada " (mixed: mixes the characteristics of the rainbow and the spider web) for example,.

Synthetic varieties of obsidian are found in the gem trade. Blue obsidian is an artificial transparent blue glass. Blue can be found in natural obsidian, but in the form of more or less visible reflections on an opaque to very slightly translucent stone. 

In popular culture

In George R.R. Martin 's A Song of Ice and Fire, obsidian is used as a weapon that can kill the Others (or White Walkers, as they are referred to in the television version of Game of Thrones). Characters in the series refer to this mineraloid as dragonglass.

It also appears in the novel, Azteca, which was an important stone in the life of the Aztecs.

In the movie Miracle Hands, Ben Carson, while walking to school, finds a piece of obsidian. Not recognizing it and because his mother wanted her children to get into the habit of reading, he goes to the library to read about rocks, identifying it. Some time later one of his teachers brings a rock and asks the class if they can recognize it, to which Ben Carson replies that he recognizes it and that he also knows how it is formed, to which the class is surprised because Ben was not a great student back then.

In the video game Minecraft, it is one of the hardest materials, and is an important material for developing portals and enchantments.

In the animated series Steven Universe he is shown as the fusion of the four main characters of the series, he makes his debut in the chapter "Change Your Mind" and after being a mysterious character it is discovered that he is the same one that is shown carved in the temple where the entire series has developed since the beginning, his appearance is a huge being totally black with long hair and eight arms, his body is covered in magma cracks and two faces that together form a third mouth full of magma where he forges his own greatsword by placing its hilt and forging the sharp blade from the lava. He has been the only character capable of cutting the limbs of the Diamond Authority ship.

Counterfeits and confusion

Because obsidian is found in relatively large quantities as a gemstone, and its price is therefore comparatively low, it is rarely counterfeited. It is also easily identified by its characteristic vitreous luster. However, black obsidian can be confused with black schorl (tourmaline group) and onyx (or colored agate) if it is not transparent. All other obsidian varieties are unmistakable due to their characteristic patterns and play of colors.

Obsidian can easily be confused with impact melt rocks, which are formed by the melting and rapid cooling of rock following a meteorite impact. Pitchstone is very similar to obsidian in appearance and formation.

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