Starry sky obsidian from Mexico, completely polished.

Obsidian is a naturally occurring volcanic glass. The name is derived from a Roman named Obsius, who in ancient times is said to have found an obsidian-like stone in Ethiopia. Obsidian is formed when lava with a maximum water content of 3-4% cools rapidly. If the content of volatile components (water, carbon dioxide) is higher, pumice stone is formed, because the released gases act as a blowing agent. The formation of volcanic glass is highly dependent on the viscosity of the liquid lava, which in turn is strongly determined by the silicic acid content. The higher this content, the slower the liquid flows. Most obsidians have a silicic acid content of 70% or more and are therefore considered rhyolites. Less commonly, trachitic, andesitic and phonolithic obsidians are also found that have lower silicic acid content. The color of obsidian varies quite dramatically depending on the contaminants it contains. The color is usually dark green to deep black, although granite with a similar composition is a light-colored material. This is usually caused by particularly finely divided hematite or magnetite. Snowflake obsidian contains inclusions of spherically grown structures, mostly of feldspar or cristobalite, called spherulites. They arise from spherical crystal growth from a single central growth core. Obsidian is a metastable material that reverts to its crystalline, more stable form over time. Volcanic glass is therefore not known from the Paleozoic and Precambrian. In the Stone Age, obsidian was a coveted raw material because it forms razor-sharp cutting edges. It was used for knives and weapons. Nowadays it is a popular material for art objects and is also used as a semi-precious stone in jewelry. It is then cut as if it were a crystal.