What is obsidian in geology ?

Obsidian is an entirely glassy, black volcanic rock with a smooth, conchoidal fracture, often marked by bubble trails. Its composition is that of a rhyolite, sometimes of a trachyte. By extension we call obsidian any massive acid to intermediate rock consisting of more than 80% glass. Its name was given by Pliny, in homage to Obsius, a character from Ancient Rome who would have discovered it. Linguists rather link this name to the Latin obsidio, "encircled" (the conchoidal breaks in obsidian forming rings).

The lavas from which obsidians originate have a high silica content, which gives them a high viscosity. This high viscosity prevents atomic diffusion through the lava, which inhibits the nucleation of the crystals. Coupled with rapid cooling, the result is a natural glass formed from lava. Obsidian when cooled is metastable on the Earth's surface (over time, glass devitrifies, crystallizing into fine-grained minerals), obsidians older than the Miocene (20 million years old) are rare.

Pure obsidian usually has a very dark appearance, although the color will vary depending on the impurities present. Iron and other transitional elements can give obsidian a dark brown to black color. Most black obsidians contain nano-inclusions of magnetite, an iron oxide. In some stones, the inclusion of small radially clustered white crystals (spherulites) of cristobalite and plagioclase feldspars in the black glass produces a mottled pattern (we speak of snowflake obsidian or flowering obsidian if the cockades are pinkish-brown). Obsidian can also contain gas bubbles inherent in the lava flow, aligned as the molten rock flowed. These bubbles can produce interesting effects such as a golden or silver glow (gold and silver obsidian). An iridescent glow, similar to that of a rainbow (fire obsidian) is caused by inclusions of magnetite nanoparticles creating thin-film interference. Colorful and striped rainbow obsidian (Celestial Eye Obsidian) from Mexico contains oriented rods of hedenbergite, which cause the effects of rainbow stripes by thin-film interference.

Obsidian is not synthesized because even if it is a relatively rare material on the surface of the globe, its deposits are very productive, which makes this material very affordable. Despite a great abundance on the market, glasses with varied and often unreal colors (water green, cobalt blue, etc...) are sold by Chinese wholesalers under the name "obsidian". They are in reality industrial glasses that are absolutely not natural... They are found in all the usual forms : spheres, eggs, tumbled stones, prisms, etc...