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Obsidian (volcanic glass)
Current inventory:  3 gems
 

Chemistry:  70 - 75% SiO₂ + MgO, Fe₃O₄

Prehistoric discovery;   IMA status: Not valid
The name Obsidian is supposedly from its discovery in Ethiopia by a man named Obsius.

Classification

Mineral Classification:

Amorphous (glass)

Strunz 8th Ed. ID:

n/a

Crystal Data

Crystallography:

Amorphous, no crystals

Crystal Habit:

n/a

Twinning:

n/a

Physical Properties

Cleavage:

Conchoidal

Fracture:

Conchoidal

Tenacity:

Brittle

Moh's Hardness:

5.5 - 6.0

Density:

2.33 - 3.00 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Optical Properties

Color:

Black, Gray, Blue, Dark Green, Red

Transparency:

Transparent to Translucent

Luster:

Vitreous (glassy)

Refractive Index:

1.48 - 1.51  Isotropic

Birefringence:

0.000   (Isotropic)

Dispersion:

n/a

Pleochroism:

n/a

Occurances

Geological Setting:

Produced from volcanoes when certain types of lava cools rapidly by flowing into a body of water.

Type Locality:

n/a

Year Discovered:

Prehistoric

View mineral photos:

Obsidian Mineral Photos and Locations

More Information

Mindat.org

Obsidian, also known as volcanic glass, is a rock rather than a mineral that is a mixture of cryptocrystalline grains of silica minerals in a glass-like suspension. It is produced from volcanoes when certain types of lava cools rapidly by flowing into a body of water. Obsidian consists mainly of silicon dioxide. Obsidian is mineral-like, but not a true mineral because it has no crystal structure so it is amorphous. Otherwise, it is very similar in composition to granite and rhyolite. It is sometimes classified instead as a mineraloid. Obsidian is typically opaque black with occassional swirls of brown and not desirable for faceting. Rare occurances of transparent blue and green Obsidian have been found in various locations. These specimens make beautiful faceted gems with intense Sapphire blue and Tsavorite Garnet green colors. Obsidian is relatively soft (5.5 - 6.0) and brittle so usually not suitable for jewelry.

Green Obsidian:
This mysterious gem has been called many names including Green Obsidian, African Moldavite, Tanzanian Tektite, Green Volcanic Glass and even just glass. One source says the Tanzanian natives call it Obsidianite. It is reported to be found in alluvial deposits and in a river bed at the base of Mt. Kilamanjaro. Some say it is a natural volcanic glass, some argue that it is man-made glass, others claim it is of meteoric origin similar to Moldavite. Calling it "African Moldvite" is incorrect however, since Moldavite only comes from the Moldau River reqion of the Czech Republic. Where ever this material comes from or however it was formed, it is a stunning gem of the finest Tsavorite green color.
 

  
Obsidian gems for sale: