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.
Most obsidian used in the jewelry trade is produced in the United States. Obsidian spear point: A spear point fashioned from opaque black obsidian. The best way to learn about rocks is to have specimens available for testing and examination.
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.
Note the faint bands on the outer surface. They result from mixing of the thick magma underground. The clean, black fractured surface shows why obsidian was valued by the native people for making arrowheads and other tools.
Chunks of obsidian are found far from their place of origin because of prehistoric trading. Therefore, they bear cultural as well as geologic information.
Water attacks obsidian readily because none of its material is locked up in crystals, making it prone to alteration into clays and related minerals. Like a sculptor grinding and brushing away the grit, wind and water have etched out subtle details inside this obsidian cobble. Obsidian is the best material for making stone tools. The stone doesn't need to be perfect to make useful implements. Obsidian fragments show the full range of its typical textures and colors. These chips are collectively called debitage.
They display some of the variety in obsidian's color and transparency. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content.
Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Obsidian is formed by rapidly cooling felsic lava extruded from a volcano with limited growth in crystals. It is commonly found in the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition high silica content creates a high viscosity resulting in the production of natural glass from the lava after rapid cooling.
Obsidian is rough, fragile, and amorphous, with very sharp edges breaking. It was used in the past for the manufacture of cutting and slicing equipment and was used experimentally as surgical scalpel blades.
Obsidian is mineral-like, but not a true mineral because it is not crystalline as a glass; however, it is too complex to be categorized as a mineral. It is classified as a mineraloid sometimes. While obsidian is usually dark in colour, similar to basalt-like mafic rocks, the composition of obsidian is extremely felsic.
With a similar composition, crystalline rocks include granite and rhyolite. Because obsidian is metastable on the surface of the Earth the glass forms fine-grained mineral crystals over time , no obsidian older than the Cretaceous period has been found. The existence of water speeds up this process of obsidian.
Although the newly formed obsidian has a low water content, usually less than 1 percent by weight, it is slowly hydrated to form perlite when exposed to groundwater.
How can you identify obsidian? The lack of a crystalline structure indicates that obsidian is not a true mineral and causes extreme sharpness of the fracture surfaces. Since prehistoric times, obsidian has been used in cutting tools and is still used today in surgical scalpels. Explore obsidian where cooling is rapid in the margins of lava flows. Glass Buttes in central Oregon is one of the best places to find obsidian in the U. Pieces of fist size can be found on the surface in abundance here.
It has a distinctive appearance of smooth glass. Obsidian is a frozen liquid that contains small amounts of mineral impurities.
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