Obsidian "silver sheen" "eyes-chain" from Mexico

Obsidian is a naturally occurring volcanic glass. The name is derived from a Roman named Obsius and an obsidian-like stone was found in Ethiopia in antiquity. Obsidian is created at a rapid cooling of lava with a water content of up to 3-4%. If the content of volatile components (water, carbon dioxide) is higher arises pumice, because the released gases act as a blowing agent. The formation of volcanic glass is highly dependent on the viscosity of the liquid lava and this is in turn strongly influenced by the content of silicic acid. The higher this content, the slower the liquid flows. Most obsidianen have a silica content of 70% or more and, therefore, be counted among the ryolieten. Less often also trachitische, andesitic and fonolithische obsidianen found that a lower silica content. The color of obsidian varies quite widely, depending on the impurities it contains. The color is most dark green to deep black, although granite with a similar composition is a light colored material. Usually the cause of particularly finely divided hematite or magnetite there. Sneeuwvlokobsidiaan contains inclusions of spherical grown structures, most of feldspar or cristobalite, which are called spherulites. They arise from spherical crystal growth from a single central urban growth. Obsidian is a meta-stable material which falls back on the long term, in its crystalline, more stable form. Volcanic glass is not known from the Paleozoic and Precambrian. In the Stone Age was obsidian is a coveted commodity as the razor-sharp cutting edges. It was used for knives and weapons. Today it is a popular material for art objects and is also used as a gemstone in jewelry. It is then cut as if it is a crystal.