Obsidian 32 Pieces Raw Minerals Stones Rocks Collecting

Only 1 Euro for Piece ! Lot 500 gr Obsidian 32 Pieces Raw Minerals Stones Rocks for Collection.
Each stone: 20-80 gr, cm 1-8.
The lot is just that of the photo.
Also available in single pieces, at this link.

Obsidian is a volcanic glass composed of 75% silicon dioxide which is formed by the rapid cooling of acidic effusive rocks. The lithotype is associated with castings with rhyolitic to basaltic chemistry. The silicate ions of which it is composed during cooling, failing to achieve the ordered formation of a crystalline lattice, take on a chaotic arrangement (amorphous structure) as in a superviscuous liquid. Obsidian is therefore a natural glass, very similar to that of human production. Very sought after in ancient times for the manufacture of sharp tools (mainly knives) it was one of the commodities that animated the trade of the Mediterranean and sustained the economic life of the places of extraction, such as Sardinia, Lipari or Pantelleria, in the prehistoric period. Today it is still worked in small decorative objects, jewelry, scalpel blades.
Pliny called it lapis obsianus or obsidianus in honor of a certain Obsius or Obsidius, who cited the stone first in some areas of Ethiopia. Some findings suggest the knowledge of stone in ancient times: the nuragic civilization used it mainly to build lances and arrows and tools of various kinds, the ancient Egyptians used obsidian to make scarabs and seals while in Central America it was used by pre-Columbian civilizations . The Aztecs (like any civilization that had them locally) used obsidian to get the tips of the arrows, knives and razors. The technique for making sharp objects from obsidian pebbles is called chipping and consists in removing chips by means of the percussion.
Even today, scalpel blades are produced in obsidian, which some surgeons prefer compared to steel for the sharpest cut and the absence of metallic particulate, a possible cause of allergy.
The obsidian can be cut in cabochon or sphere for precious necklaces.
The locations are: Hungary, Iceland, Italy (Pantelleria, Lipari island of Palmarola, Sardinia), Greece (Island of Melos, Island of Giali), United States, Mexico, Peru, Brazil, Turkey, Java (Indonesia), Japan , Ethiopia, Djibouti.
Obsidian is currently used to make rock wool; a sort of glass in very thin fibers obtained by melting obsidian at 1300 ° C, and then to create very precious necklaces.

Chemical formula: silicates.
Crystallographic properties: amorphous crystalline group and amorphous crystalline system.
Physical properties:
Density 2-3 g / cm³
Hardness (Mohs) 5-6
No flaking
Concoid fracture
Dark gray color
Glass shine
Opacity from transparent to opaque
White-yellowish smear
Relatively common dissemination.