Snowflake obsidian from Utah, USA. (~17.3 centimeters across at its widest)
Igneous rocks form by the cooling and crystallization of hot, molten rock (magma and 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 readily identifiable. It 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 sometimes referred to "glassy rhyolite".
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 that formed the first way is along the margins of basaltic lava flows at Kilaeua Volcano (Hawaii Hotspot, central Pacific Ocean) - such material is also known as "tachylite". The obsidian sample seen here formed the second way.
Obsidian is unstable on geologic time scales - it will slowly convert to material that is not obsidian. A partially-converted obsidian is a distinctive rock called snowflake obsidian. The black portions of the rock seen here are rhyolitic obsidian (glass). The white patches ("snowflakes") are devitrification spots composed of cristobalite (SiO2, a polymorph of quartz).
Locality: unrecorded / undisclosed site attributed to Twin Peaks, Utah, USA
