Black obsidian with streaks of mahogany obsidian
Obsidian (the bluish coloration shown above is an artifact of reflecting scanner light)
Igneous rocks form by the cooling & crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.
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 generally 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 at the Yellowstone Hotspot (northwestern Wyoming, USA) and Big Obsidian Flow at the Newberry Volcano (central Oregon, USA).
Obsidian is moderately hard, has conchoidal fracture (smooth and 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 a 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 (see example above).
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). The obsidian sample shown above formed the second way.