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Obsidian: Characteristics And Properties

Obsidian is a natural glass formed by the meeting of lava from an erupting volcano with cold air or water, imbuing obsidian with all four elements: earth, fire, water, and air.

Obsidian is an ancient rock that was discovered in Ethiopia by a Roman explorer named Obsidius.

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What is Obsidian?

Obsidian, or lapis obsidianus, is essentially dark glass. A type of igneous rock, called extrusive igneous rock, obsidian is extruded from the Earth as lava which when rapidly cooled forms a natural glass.

As a volcanic glass, obsidian is mineral-like, or mineraloid, but is not a true mineral. This is because glass is amorphous, not crystalline in structure. An amorphous solid is not highly ordered in internal structure, unlike a crystal. Obsidian is primarily made of silica, not unlike black tourmaline, but due to the fact that the obsidian forms quickly from the cooling lava, there is no time for a crystalline structure to form, resulting in the amorphous black glass called obsidian.

Many variations of obsidian exist, and their appearance depends on the inclusion of impurities, such as iron which leads to dark brown or black obsidian. When the mineral cristobalite is encased in the obsidian, it results in a rock with mottled white patches in the dark obsidian as seen above in the example of snowflake obsidian.

How is Obsidian Formed?

When a volcano erupts and lava gushes forth to abruptly meet with a cooling element. Obsidian can be formed in several different locations around lava. These locations include the edge of a lava flow or a lava dome as viscous lava slowly pours out of the mouth of the volcano. Obsidian can also be formed when the lava flow meets water or solidifies in the air. In order to form obsidian, the lava must cool quickly, without time to crystallize into a structured material, resulting in amorphous solid glass.

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Obsidian forms wherever there are volcanoes that emit lava that is rich in silica content, called felsic or rhyolitic lava. Obsidian can be found throughout the Americas, Australia, and Europe. Because obsidian is not the most stable rock as it can be worn down by natural elements, it is rare to find obsidian that is more than a few million years old.

What Are the Properties of Obsidian?

PropertyValue
ColorDark brown, green, or black
LusterVitreous (glass-like)
TransparencyTranslucent
Crystal SystemAmorphous
Chemical CompositionPrimarily silica (silicon dioxide, SiO2)

Impurities can include iron oxide, magnetite nanoparticles, and minerals

Density (g/cm3)2.4 (average)
Hardness (Mohs Scale)5 – 6
Fracture TypeConchoidal

Historical Uses

The fracture type of obsidian is conchoidal. This means that when the obsidian is struck and breaks, it does so with smooth, razor-sharp edges and curved surfaces. This type of break occurs because obsidian is brittle, and an amorphous material that has no natural stress lines or planes of separation. The conchoidal fracturing type of obsidian makes the rock an ideal material for weaponry and tools as flakes of obsidian can be chipped away leaving behind an incredibly sharp edge.

Obsidian has been used to make arrowheads and spearheads, as seen in the figure below. Knives and scrapers made of obsidian have also been found in archaeological sites. Ancient Mesoamericans manufactured a type of sword called a macuahuitl which was made of wood and obsidian that could be used in slashing cuts to inflict wounds with the sharp and serrated edge.

Obsidian was not always used as a practical tool or weapon. It was also used for aesthetics. In some cases, larger pieces of obsidian were polished as mirrors. Obsidian has also been used to make carved masks or statues. Largely obsidian was used as a decorative ornament in various forms of jewelry. Beads can be made from obsidian, as well as earrings and brooches.

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Obsidian has also been used as a sort of back plating for pieces of opal jewelry, where a thin slab of obsidian is placed behind a piece of opal to add a dark background and provide better contrast for the rainbow hues of the opal.

Modern Uses

Obsidian is still used today in the making of decorative pieces such as jewelry and various carvings. Crystal balls made out of obsidian are popular in mystic settings. Support stands for turntables have been made out of obsidian since the 1970s. More interestingly, obsidian is still used in surgical practices.

While not approved by the FDA for use in human surgery, scalpel blades may be made from obsidian and have been used in experimental animal surgeries. Obsidian blades can be cut to a thickness of three nanometers (ten times thinner than common razor blades) with a very fine edge. When a blade is made of surgical steel, due to the regularity of the microscopic structure of the material, the edge of the blade is always slightly rough no matter how sharp.

Obsidian, on the other hand, produces a fine and smooth edge that is highly sharp (many times sharper than a surgical steel scalpel). Incisions made to the skin using an obsidian blade have been shown to cause finer cuts with less trauma caused to the tissue and lower rates of inflammation resulting in reduced scarring. Incisions made with surgical steel blades tend to appear more saw-like tears while obsidian blade cuts are smooth and precise.

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