File:Obsidian 32.jpg

English: Obsidian is a glassy-textured, extrusive igneous rock. Obsidian is natural glass - it lacks crystals, and therefore lacks minerals. Obsidian is typically black in color, but most obsidians have a felsic to intermediate chemistry. Felsic igneous rocks are generally light-colored, so a felsic obsidian seems a paradox. Mafic obsidians are scarce, but they are also black and glassy.

Obsidian is an uncommon rock, but can be examined at several famous localities in America, such as Obsidian Cliff at the Yellowstone Hotspot (northwestern Wyoming) and Big Obsidian Flow at Newberry Volcano (central Oregon).

Obsidian is moderately hard, has a conchoidal fracture (smooth and curved fracture surface), and sharp broken edges. Freshly-broken obsidian has the sharpest edges of any material known, natural or man-made (as seen under a scanning electron microscope).

Obsidian forms two ways: 1) very rapid cooling of lava, which prevents the formation of crystals; 2) cooling of high-viscosity lava, which prevents easy movement of atoms to form crystals. An example of obsidian that formed the first way is the very top portion of basaltic lava flows at Kilaeua Volcano (Hawaii Hotspot, central Pacific Ocean) [however, most geologists don't call such material "obsidian"]. The obsidian sample seen here formed by cooling of high-viscosity lava.