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Uses of Obsidian as a Cutting Tool
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…
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Occurrence of Obsidian
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. Significant deposits of obsidian are found in Argentina, Canada, Chile, Ecuador, Greece, Guatemala, Hungary,…
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Are There Other Glassy Igneous Rocks?
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.
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What is the Composition of 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.”
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What Color is Obsidian?
Rainbow Obsidian: A baroque cabochon of iridescent “rainbow 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…
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Where Does Obsidian Form?
Obsidian is usually an extrusive rock – one that solidifies above Earth’s surface. However, it can form in a variety of cooling environments:
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What is Obsidian?
Obsidian is an igneous rock that forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. It is an amorphous material known as a “mineraloid.” The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture (see photo).
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Uses of Granite
Granite is the rock most often quarried as a “dimension stone” (a natural rock material that will be cut into blocks or slabs of specific length, width, and thickness). Granite is hard enough to resist abrasion, strong enough to bear significant weight, inert enough to resist weathering, and it accepts a brilliant polish. These characteristics…
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When “Granite” Is Inadequate
So, the name “granite” is a name used for igneous rocks that are composed of orthoclase feldspar, quartz, plagioclase feldspar, micas, and amphiboles that are present in crystals large enough to be visible with the unaided eye. That name isn’t specific enough for some purposes and for some people. Special names are used for granitic rocks based…
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Multiple Definitions of Granite
The word “granite” is used in a variety of ways by different people. A simple definition is used in introductory geology or earth science courses. A more precise definition is used by petrologists (geologists who specialize in the study of rocks). And, the definition of granite expands wildly when used in the crushed stone and dimension stone…