Obsidian is an easily recognizable igneous rock. It is a glassy-textured, extrusive igneous rock. Obsidian is a natural glass - it lacks crystals, and therefore lacks minerals. Obsidian is typically black in color, but most obsidians have a felsic chemistry. Felsic igneous rocks are light-colored, so a felsic obsidian seems a paradox. Mafic obsidians are scarce, but they have the same appearance.
Obsidian is an uncommon rock, but can be examined at several famous localities in America, such as Obsidian Cliff in the Yellowstone Caldera (northwestern Wyoming, USA) and Big Obsidian Flow in the Newberry Volcano Caldera (central Oregon, USA).
Obsidian is moderately hard, has conchoidal fracture (smooth & curved fracture surface), and has exceedingly sharp edges. Freshly-broken obsidian has the sharpest edges of any material known, natural or man-made (as seen under scanning electron microscope).
The most common color variation of obsidian is brownish-red, which occurs throughout the rock or as streaks in a black obsidian. Such rocks are called mahogany obsidians (see below). A rare variety is rainbow obsidian (see below).
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 along the margins of basaltic lava flows at Kilaeua Volcano (Hawaii Hotspot, central Pacific Ocean). Examples of obsidian that formed the second way are shown below.
Black obsidian with streaks of mahogany obsidian.
Velvet rainbow obsidian from Mexico.