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Gemstones Facts

 

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Obsidian Obsidian Rock

Mohs scale hardness: 5-5.5

Obsidian is a natural glass and is formed from volcanic lava which cooled too fast for significant crystallization to happen. Due to the instant cooling, very few crystals can occur in the obsidian. That is why obsidian rocks are amorphous and they dont have any crystal system. It is very similar in composition to rhyolite and granite.

If granite and rhyolite, both of which were originally molten and are similar in composition to obsidian, then why is obsidian glassy? The original cooling rate and water content of the magma is the answer. Granite cools very slowly miles below the earth's surface. The formation of sizable mineral crystals is allowed from this slow cooling over million of years within the slowly cooling mass of molten rock. Rhyolite contains smaller mineral crystals than granite because it cools more rapidly near the surface of the earth. When this magma of rhyolite approaches the surface of the earth and the pressure of burial decreases, most of the water content in the magma is lost as steam. With little remaining water, this resulting magma that is rich in silica becomes very viscous (thick and pasty) obsidian magma. It is so viscous that sizable mineral crystals canot grow before chilling of the magma "freezes" crystal development.

Obsidian Chemical Properties

Although obsidian is mineral-like, it is not a true mineral or rock because there is no crystalline structures contained in it and the composition is too complicated to comprise a single mineral. Sometimes it is considered as a mineraloid, mineraloid means a mineral-like substance which lacks crystal structures like pearls or opals.

Obsidian’s color varies depending on the presence of impurities. Magnesium and iron typically give the obsidian a dark green to black color. Obsidian often occurs in black colour and it is often shiny even in unpolished form. Obsidian is composed of 70% silicon dioxide or more, and when mixed with certain minerals its colour will change. Obsidian possesses the peculiar property of presenting different apearance according to the manner in which it is cut. When it is cut in one direction, it is of a beutiful jetty black, and when cut across another direction it is glistering gray. Like all glass and certain types of natural rocks, obsidian breaks with a characteristic conchoidal fracture. It may have been polished to create early mirrors.

Because obsidian is metastable at the surface of the earth (over time the glass becomes fine-grained mineral crystals) and no obsidian has been found that is older than Cretaceus age. The presence of water will accelerate the breakdown of this obsidian. A pure obsidian rock is translucent and dark in color. It has low water content when fresh (less than 1% by weight), but becomes progresively hydrated when exposed to groundwater, forming perlite.

Obsidian has similar properties to quartz because of similar chemistry, therefore obsidian is often confused with smoky quartz. A notable difference is alteration or absence of the crystal properties in obsidian, which are prominent in quartz. In obsidian rocks, the color distribution is also more even than smoky quartz. Classified under amorphous rocks and its hardness is much lesser than quartz. If compared to other ignoeus rocks like pumice, obsidian rocks are devoid of large voids. Obsidian rocks are lack of crystal faces, distinctive flow bubles, conchoidal fracture upon breakage and presence of other igneous rocks.

Types of Obsidian Rock

Based on several attributes, obsidian rocks are divided into many types: Snowflake obsidian is a common black type of obsidian with inclusion of white, small, radially clustered crystals of cristobalite which produce a blotchy or snowflake pattern. Obsidian may contain bubbles of air remaining from the lava flow, that are aligned along layers createdd as the molten rock was flowing before being cooled. All of these bubbles can produce effects such as a rainbow sheen, known as Rainbow Obsidian or a golden sheen, known as Sheen Obsidian. Small nuggets of obsidian that have been naturally smoothed and rounded by water and wind are called "apache tears."

Uses of Obsidian

Due to its attractive glass like texture, obsidian is used as semiprecious stones. During the Stone Age obsidian was a favored material for creating weapons and it is still used for some surgical instrument today. It is also kept as decorative specimens, especialy after designs are carved over the surface. It is also used in making surgical blades, gemstones, crystal healing and ornamental application. Even the sharpest knife made of metal has jagged, irregular blade when viewed under a strong microscope. Obsidian blade is still even and smooth when examined under an electron microscope, thus giving smooth cuts.

Obsidian was discovered by Obsius in Ethiopia, therefore the gemstone is named after him. Obsidians can be found in Japan, Hawaii, Iceland, Mexico, Hungary, Guatemala, Ecuador, New Mexico and Arizona.

It is believed obsidian have special properties:

Obsidian is an alternate birthstone for those who born in December.
Astrologically, obsidian is associated with the zodiac sign of Sagittarius.

 

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