Geology And Geophysics

Why not all Obsidian is Completely Black

Brooke Vonbank's image for:
"Why not all Obsidian is Completely Black"
Image by: 

What is obsidian?

Obsidian can be found in a variety of colors; however, black is by far the most commonly occurring variety. Being an extrusive igneous rock, meaning it forms above earth's surface from the solidification of magma, obsidian forms when the magma that reaches the earth's surface cools very rapidly, preventing the formation of crystals. This rapid cooling creates a glossy texture, resulting a resemblance of opaque glass. The color and pattern of obsidian that is formed when the magma cools, all depends on the minerals present in the mixture. Streaks or stripes may be visible due to insufficient time for the magma to mix with various minerals during its cooling period. As this rock is formed from magma flows, deposits are only commonly found in locations that have had past volcanic activity. It is believed that obsidian symbolizes self control, protection, and blocks negativity from the mind.

Colors and associated justifications:

Traditionally, obsidian takes on a glossy jet black appearance, which is the result of the mixture of magnetite, silica, hornblende, biotite, pyroxene, plagioclase, and other small fragments rocks present.

A Red or brown hue may result of small traces of hematite or iron oxide in the rock formation.

Iridescent or metallic appearance occurs from small amounts of trace minerals, debris, or gas which is evenly distributed throughout the rock.

Occasionally, obsidian may form as two colors swirled together. This occurs from the uneven mixing of elements and minerals in the obsidian, due to lack of "mixing" time during the formation process.

A rainbow appearance may result from various crystal formations of the feldspar family located throughout the rock.

A gold or silver shine may appear as the result of the small gas bubbles within the rock being stretched flat as the magma is folded during the formation process.

Being clear or partially clear is a rare quality in obsidian, as volcanic ash and many minerals and elements usually cause impurities in the obsidian as it forms. However is the mixture has few to no opaque impurities, the rock will most likely appear translucent.

White or grey clusters may form throughout the obsidian if the obsidian begins to crystallize, which may occur during formation, or later as time passes. This may be a result of the unstable chemical composition of obsidian, which also leads to the relatively short life of the rock, not lasting more than a few million years.

More about this author: Brooke Vonbank

From Around the Web