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. Mafic glass is usually called tachylite (if black) or sideromelane (if golden-brown).

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, USA) and Big Obsidian Flow at the Newberry Volcano (central Oregon, USA).

Obsidian is moderately hard and has a conchoidal fracture (smooth and curved fracture surface), with 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 (tachylite) that formed the first way is along the margins of basaltic lava flows at Kilaeua Volcano (Hawaii Hotspot, central Pacific Ocean). Most obsidian formed the second way.

Seen here is rainbow obsidian, a rare variety. The rainbow effect is two-beam interference coloration (see: www.flickr.com/photos/jsjgeology/albums/72157672953170470/ ). Two-beam interference colors in Mexican rainbow obsidian occur due to the presence of flow-aligned microphenocryst rods in the glass (Ma et al., 2001). Microphenocrysts can be hedenbergite pyroxene (CaFeSi2O6) or plagioclase feldspar (NaAlSi3O8 to CaAl2Si2O8).

Locality: unrecorded / undisclosed site in Jalisco State, Mexico (but possibly from the La Revoltosa Mine at Mount San Andreas, east of the town of San Andrés, Magdalena Municipality, northwestern Jalisco State, western Mexico)

Reference cited: