It is an amorphous material known as a " mineraloid. Obsidian is usually an extrusive rock - one that solidifies above Earth's surface. However, it can form in a variety of cooling environments:.
Types of Obsidian: The specimens shown above are from Glass Butte rockhounding site in central Oregon. It shows the diversity of obsidian types that can be found in a small geographic area. Clockwise from upper left are: double flow obsidian, rainbow obsidian, black obsidian, pumpkin obsidian, mahogany obsidian, gold sheen obsidian, and the piece in the center is gold sheen. Mahogany obsidian: A tumble-polished specimen of "mahogany obsidian.
Black is the most common color of obsidian. However, it can also be brown, tan, or green. Rarely, obsidian can be blue, red, orange, or yellow. The colors are thought to be caused mainly by trace elements or inclusions. Occasionally two colors of obsidian will be swirled together in a single specimen.
The most common color combination is black and brown obsidian swirled together - that's called "mahogany obsidian" see photo. As a "glass," obsidian is chemically unstable.
With the passage of time, some obsidian begins to crystallize. This process does not happen at a uniform rate throughout the rock. Instead it begins at various locations within the rock. At these locations, the crystallization process forms radial clusters of white or gray cristobalite crystals within the obsidian.
When cut and polished, these specimens are referred to as "snowflake obsidian" see photos. Rarely, obsidian has an iridescent or metallic "sheen" caused by light reflecting from minute inclusions of mineral crystals, rock debris, or gas. These colored specimens are known as "rainbow obsidian," "golden obsidian," or "silver obsidian," depending upon the color of the sheen or iridescence.
These specimens are very desirable for the manufacture of jewelry. Snowflake obsidian: A tumble-polished specimen of "snowflake obsidian. Most obsidians have a composition similar to rhyolite and granite. Granites and rhyolites can form from the same magma as obsidian and are often geographically associated with the obsidian. Rarely, volcanic glasses are found with a composition similar to basalt and gabbro.
These glassy rocks are named "tachylyte. Pumice , scoria , and tachylyte are other volcanic glasses formed by rapid cooling. Pumice and scoria differ from obsidian by having abundant vesicles - cavities in the rock produced when gas bubbles were trapped in a solidifying melt.
Tachylyte differs in composition - it has a composition similar to basalt and gabbro. Obsidian outcrop: Obsidian along the edge of a lava flow in central Oregon.
Obsidian knife blade: A knife blade manufactured from mahogany obsidian. The craftsman who made this blade had a very high skill level and was able to produce a serrated edge. 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. In the United States it is not found east of the Mississippi River, as there is no geologically recent volcanic activity there. Rhyolite typically cools more rapidly near the earth's surface and contains smaller mineral crystals than granite. When rhyolite magma approaches the earth's surface and the pressure of burial decreases, most of the water in the magma is lost as steam.
The resulting silica-rich magma with little remaining water becomes very viscous thick and pasty obsidian magma. This magma is so viscous that sizable mineral crystals cannot grow before chilling of the magma "freezes" crystal development. Some obsidian is erupted as lava flows at the ground surface.
These surface flows are so viscous that they flow very slowly. One article I read indicated that "an ant could probably outrun an obsidian lava flow. Portions of this obsidian flow are mixed with layers of pumice, a glassy, bubble-rich, lightweight rock that develops when water vapor steam escapes rapidly from the molten glass at or near the ground surface.
Sometimes obsidian of excellent quality develops as surface lava flows. However, the best quality obsidian often forms below the ground surface around volcanic vents. Silica-rich magma squeezes into rock fractures to form layers and lenses of obsidian that are relatively free of dirt, ash and other impurities. Changes in magma composition and water content often occur during the eruption and subsurface emplacement of obsidian flows. The high viscosity of the molten obsidian prevents effective mixing of these magmas, resulting in obsidian that is "streaked" with different layers or colors.
Each of these streaks or lines may represent a distinct pulse of an obsidian eruption. You can visualize the process that results in streaked obsidian if you consider two blobs of green and red taffy a viscous candy that are mixed together.
Distinct streaks of red and green taffy result as the blobs are mixed. In the case of obsidian, the slow flow of stiff, viscous magma away from the source vent provides the mixing needed to create the layered or streaked varieties of obsidian. The "midnight lace" variety of obsidian often has incredibly contorted streaking, apparently formed as the obsidian layers were stretched and rolled with slow movement of the magma. The various colors of obsidian are a result of several factors.
Clear varieties of obsidian contain very few opaque impurities or microscopic mineral crystals. Red or brown obsidian generally results from tiny crystals or inclusions of hematite or limonite iron oxide. Abundant, microscopic crystals of minerals like magnetite, hornblende, pyroxene, plagioclase and biotite, combined with tiny fragments of rock, likely produce the jet-black varieties of obsidian.
Microscopic crystals of various types of feldspars may yield the unique blue, green, purple or bronze colors associated with rainbow obsidian. The reflectance of rainbow obsidian is likely attributed to a preferred orientation of microscopic crystals of feldspar or mica oriented along flow layers.
A certain amount of water always is present in obsidian. Very small inclusions of water vapor in the form of bubbles often are trapped in the glass. Tiny gas bubbles that have been stretched nearly flat along the flow layers in obsidian generally cause the reflectance of gold sheen and silver sheen obsidian.
Some of these bubbles are visible to the naked eye. The bubbles can be seen readily with a strong magnifying glass or a microscope. Obsidian was valued in Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads. Like all glass and some other types of 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 artefacts. Obsidian is used as a cutting tool because of its conchoidal fracturing where it breaks into thin sheets and have sharp edges. In stone age obsidian was used as a cutting tool for making any sharp tool and it is still used as a cutting tool in modern surgeries. It is also used as a jewellery by making beads of obsidian.
The conchoidal fracture of obsidian causes it to break into pieces with curved surfaces. This type of fracturing can produce rock fragments with very sharp edges.
These sharp fragments may have prompted the first use of obsidian by people. The first use of obsidian 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.
Obsidian was used to make knives, arrowheads, spear points, scrapers, and many other weapons and tools. Once these discoveries were made, obsidian quickly became the raw material of preference for producing almost any sharp object.
The easy-to-recognise rock became one of the first targets of organised "mining. Although using a rock as a cutting tool might sound like "stone age equipment," obsidian continues to play an important role in modern surgery. Obsidian can be used to produce a cutting edge that is thinner and sharper than the best surgical steel. Today, thin blades of obsidian are placed in surgical scalpels used for some of the most precise surgery. In controlled studies, the performance of obsidian blades was equal to or superior to the performance of surgical steel.
Obsidian is a popular gemstone. It is often cut into beads and cabochons or used to manufacture tumbled stones. Obsidian is sometimes faceted and polished into highly reflective beads. Some transparent specimens are faceted to produce interesting gems.
The use of obsidian in jewellery can be limited by its durability. It has a hardness of about 5. It also lacks toughness and is easily broken or chipped upon impact.
These durability concerns make obsidian an inappropriate stone for rings and bracelets. It is best suited for use in low-impact pieces such as earrings, brooches, and pendants.
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