Obsidian

Specific Gravity: &nbsp;2.35
Refractive Index: &nbsp;1.48-1.51
Birefringence (DR): &nbsp;None
Luster: &nbsp;Vitreous
Crystal Structure: &nbsp;Amorphous
Composition: &nbsp;Mainly silicon dioxide
Hardness: &nbsp;5

Obsidian is a natural glass. It is formed from volcanic lava that cooled too quickly for significant crystallization to occur. Hence it is amorphous, with no cleavage and a conchoidal fracture. Obsidian is usually black, but brown, grey, and more rarely red, green, and blue stones are also found. The color may be uniform, striped, or spotted.

Some inclusions give the obsidian a metallic sheen, while internal bubbles or crystals (called crystallites) produce a ‘snowflake’ effect (hence snowflake obsidian) or an iridescence seen as flashes of color.

Obsidian has been used since prehistoric times for making tools, weapons, masks, mirrors, and jewelry. The very sharp shards of natural glass have been fashioned as blades, arrowheads, and daggers. Today most obsidian jewelry comes from Central and North America.

Polished obsidian has a smooth and glassy surface. On the other hand, a rough stone has an uneven surface. The stone has mineral-lined cavities, called spherules. The rare red obsidian may display banding caused by the solidification of flowing lava.

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Occurrence

Obsidian is found in areas where there is or has been volcanic activity. For instance, Java, Japan, and Hawaii (USA) are the prime locations. Obsidian also occurs in Guatemala, Ecuador, Mexico, the USSR, Hungary, Iceland, the Lipari Islands off Italy. Dark nodules are found in New Mexico and Arizona (USA).