Well tumbled Natural Snowflake Obsidian  Stone (small to medium size pieces) - 0.5 Kg Lot (1.10 pound)

Weight per piece: 0.6 to 8.5 gram

Sizes:  7x11x11 mm to14x15x15 mm (0.3x0.4x0.4 to 0.5x0.6x0.6 inch)

Quantity: about 75 to 150 pieces in each lot

These stones are from Africa.

The pictures in the listing are just for reference as we are selling multiple pieces with the same pictures.

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Obsidian

Category  Volcanic glass

Chemical formula  70–75% SiO2, plus MgO, Fe3O4

Color  Black

Fracture  Conchoidal

Mohs scale hardness  5–6

Luster  Vitreous

Specific gravity  ~2.4

Optical properties  Translucent

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.

It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerization degree of the lava. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Obsidian is hard and brittle; it therefore fractures with very sharp edges, which had been used in the past in cutting and piercing tools, and are still used as surgical scalpel blades.

Obsidian is the rock formed as a result of cooled magma, which is the parent material. Having a low water content when newly formed typically less than 1% water by weight, becomes progressively hydrated when exposed to groundwater, forming perlite. Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis.

Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too complex to comprise a single mineral. It is sometimes classified as a mineraloid. Though obsidian is usually dark in color similar to mafic rocks such as basalt, obsidian's composition is extremely felsic. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite. Because obsidian is metastable at the Earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age. This breakdown of obsidian is accelerated by the presence of water.

Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark brown to black color. Very few samples are nearly colorless. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). It may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden sheen (sheen obsidian) or an iridescent, rainbow-like sheen (rainbow obsidian).

Obsidian can be found in locations which have experienced rhyolitic eruptions.