Ever since I had my first rock collection as a child, I've loved obsidian. Sharp and shiny, obsidian is so different from other rocks. But until a few years ago when I made my first obsidian collecting trip to Glass Buttes, Oregon, I thought obsidian was pretty much just black glass. That amazing trip really opened my eyes. The ancient volcanic hills called Glass Buttes hold a dazzling variety of gem-quality obsidian, including: mahogany, red, flame, midnight lace, jet black, pumpkin, brown, rainbow, gold sheen, silver sheen, green, lizard skin, snowflake and more.
My goal in this article is to increase your awareness of some of the more fascinating aspects of this incredible stone. Obsidian is natural glass that was originally molten magma associated with a volcano.
This volcanic glass has an almost total absence of sizable mineral crystals within the glass matrix. When I say "crystals," don't visualize those beautiful pointed prisms of quartz found in geodes.
All rocks consist of mixtures of various crystalline minerals. When crystallization occurs, the atoms that comprise a mineral become arranged in regular, geometric patterns that are unique to the specific mineral. Crystal faces form only where there is enough open space in the rock mass to allow the natural geometric forms of the crystals to develop as free faces. Granite is composed entirely of intergrown crystals of quartz, feldspar, mica and other minerals.
These relatively large mineral crystals easily visible to the naked eye give granite a rough fracture surface. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic "conchoidal" fracture. This smooth, curved type of fracture surface occurs because of the near-absence of mineral crystals in the glass.
The intersections of conchoidal fracture surfaces can be sharper than a razor. This had obvious advantages for our Stone Age ancestors, who used obsidian extensively for tool making. Obsidian consists of about 70 percent or more non-crystallized silica silicon dioxide. It is chemically similar to granite and rhyolite, which also were originally molten. Because obsidian is not comprised of mineral crystals, technically obsidian is not a true "rock.
Obsidian is relatively soft with a typical hardness of 5 to 5. In comparison, quartz crystallized silicon dioxide has a hardness of 7. Obsidian occurs only where geologic processes create volcanoes and where the chemical composition of the magma is rich in silica. Obsidian-bearing volcanoes are typically located in or near areas of crustal instability or mountain building. In North America, obsidian is found only in localized areas of the West, where the processes of plate tectonics have created geologic conditions favorable to volcanism and the formation of obsidian.
Obsidian typically forms near the end of a volcanic cycle and is often associated with domes of volcanic rock, such as the hills of Glass Buttes, Oregon. If obsidian is similar in composition to granite and rhyolite, both of which were originally molten, then why is obsidian glassy?
The answer relates to the original cooling rate and water content of the magma. 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. 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-recognize rock became one of the first targets of organized "mining.
Apache tears: "Apache Tears" is a name used for small obsidian nodules of about one inch or less that can be found in volcanic areas of the southwestern United States. Their name comes from a Native American legend. During a battle between Apaches and the U. Cavalry in , the outnumbered Apaches, facing defeat, rode their horses over a cliff rather than allow themselves to be killed by their enemy.
Upon hearing the story of the battle, the tears of their family members turned to stone when they hit the ground.
Those stones are now found as the black obsidian nodules. The thickest lava has the largest concentration of silica. Why does this matter? Since silica polymerizes magma. There are many bridges chemical connections between silica oxygen anions SiO2 , which is why it is so difficult to relocate this magma. If the water has many electrons cations , it is less viscous, because the frame system of silica is broken by these cations.
The chemicals hodium silica content produce a elevated viscosity that shapes a natural glass from lava when rapidly drying. The chemical composition is often discovered on the edges of rhyolitic lava flows recognized as obsidian flows. The absence of crystal growth is explained by the inhibition of atomic diffusion by this high-viscous lava. Obsidian is difficult, fragile and amorphous and therefore has strong corners of fractures. The instruments for slicing and piercing were previously used and were used as operative scalpel blades experimentally.
The obsidian is the rock created by fast-cooled lava, the material father. Extensive obsidian formation can happen if felsic magma quickly recools on the corners of a volcanic dome or felsic lava stream, or if lava cools during abrupt water or wind touch. Obsidian can be intrusive when felsic lava cools on the edges of a deck. It is like granite and rhyolite, which were also initially frozen, chemically.
It is actually a congealed fluid with small quantities of microscopic and impure microscopy. Obsidian with a typical hardness of 5 to 5.
In contrast, quartz silicium dioxide crystallized is of 7. Pure obsidian is generally black, although the colour differs with the existence of impurity. The jade could be light gray to black with iron and other transformation components. The majority of black obsidians are magnetite-nanoinclusions, iron oxide. Very few obsidian specimens are almost colorless.
In some rocks, the incorporation of the mineral cristobalitis in the black glass of tiny, yellow, radially grouped rocks produces a blotchy snowflaking image. Obsidian can include patterns of gas bubbles from the lava flow that align with layers created during molten rock before cooling.
These bubbles could generate exciting impacts like a golden blade obsidian blade.
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