Obsidian
Obsidian is a type of naturally occurring glass, produced by volcanoes (igneous origin) when a felsic lava cools rapidly and freezes without sufficient time for crystal growth (see glass transition temperature). It is commonly found within the margins of felsic lava flows, where cooling is more rapid. Because of the lack of crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as arrowheads, and its modern use as surgical scalpel blades.
While a rock like basalt is dark because of ferromagnesian enrichment, obsidian consists mainly of SiO2 (silicon dioxide), 70% or more. Obsidian is mineral-like, but not a true mineral because it is not crystalline.
Obsidian was highly valued in certain Stone Age cultures because, like flint, it can be fractured to produce sharp blades or arrowheads. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic conchoidal fracture. It may also have been polished to create early mirrors.
Obsidian is used in cardiac surgery, as well-crafted obsidian blades have a cutting edge up to five times sharper than high-quality steel surgical scalpels, with the edge of the blade reaching veritable molecular thinness. It produces a cleaner cut and less tissue trauma, which translates to faster healing and less scar tissue