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Obsidian Process

Obsidian is a type of volcanically formed natural glass. It is found in many areas of the world where certain types of volcanic eruptions have occurred. Obsidian was used by Stone Age cultures as a material for making tools and weapons because it can be fractured to create sharp edges. Today it is considered a semi-precious gemstone and used for jewelry.

Description
- Obsidian is shiny with a glasslike texture. Obsidian is a glass, rather than a mineral, because it lacks a crystalline structure. When it fractures, it breaks in a "conchoidal," or "shell form" fracture. A conchoidal fracture is a curved breakage that does not occur on natural planes of cleavage, due to obsidian's fine-grained composition and lack of crystals. Conchoidal fractures tend to resemble the ridges on the surface of a seashell. The intersections of these fractures result in razor-sharp edges.
Formation
- Obsidian is formed when rhyolitic (felsic) lava extrudes at or near Earth's surface during a volcanic eruption and is rapidly cooled. Obsidian-forming lava is rich in silica, giving it the felsic designation. As the magma reaches Earth's surface, the decrease in pressure causes it to lose its water content as vapor, resulting in a thick, slow-moving magma that prevents crystals from forming in the time it takes to quickly cool at the surface. This rapid cooling and lack of crystal formation is what results in the glassy structure of obsidian.
Characteristics
- Because of the steam loss in the obsidian formation process, it is low in water content, about .1 to .5 percent. It has a hardness rating of 5 to 5.5, similar to window glass. However, obsidian also has a low compressive strength of .15, meaning that when combined with its lack of crystalline structure, it is quite brittle. Obsidian is also acidic, with a specific gravity of 2.6. Most obsidian is less than 20 million years old. Older obsidian tends to devitrify, or turn from glass to rock.
Color Variations
- Most people associate obsidian with a jet-black hue. While most obsidian is black or mostly black, the presence of tiny crystals of other minerals during formation causes several color variations. Black obsidian coloration is the result of tiny iron oxide crystals. Hematite and various feldspars and the oxidation of those minerals causes red, blue, purple, green and other coloration. Obsidian is also found in streaked or banded patterns of multiple colors, blotchy "snowflake" patterns, gold or silver sheen, pumpkin and other hues.

Rocks & Minerals