Classification: Mineraloid because it is an amorphous glass and not a crystal.
It is produced when felsic lava, or rocks relatively rich in elements that form feldspar and quartz, is extruded from a volcano and rapidly cools with minimal crystalline growth. It is commonly found within the margins of ryholitic lave flows known as obsidian flows where the chemical composition induces a high viscosity and polymerization degree of the lava. Polymerization is where the molecules line up in an orderly and systematic fashion there they are in connected the same in three dimensions. The inability for atomic diffusion, or the ability of atoms to jump from one molecule to another, through this highly viscous and polymerized lava explains the lack of crystal growth. Due to the hardness and brittleness of the mineraloid, it was adapted early in history as cutting and piercing tools. Some surgical companies are utilizing obsidian scalpel blades since they are sharper than any metal blade.
Formula: 70–75% SiO2 in water plus MgO, Fe3O4
Varieties: Apache Tears- Small rounded pebbles of black obsidian
Mahogany Obsidian- variety with black and red banding
Rainbow Obsidian- Multi-color iridescence
Sheen Obsidian- Variety exhibiting a gold sheen effect.
Snowflake Obsidian- Variety containing white “snowflake” crystal patterns of the mineral cristobalite.
Fire Obsidian- Variety that has a thin layer of magnetite that diffracts the light as it passes through it. The layer of magnetite is about the thickness of a wavelength of light.
Color: Black, gray, dark green, red
Obsidian only forms around volcanoes that are ryholitic in nature. The ancient volcanic hills called Glass Buttes in Oregan hold a large variety of gem-quality obsidian, including: mahogany, red, flame, midnight lace, jet black, pumpkin, brown, rainbow, gold sheen, silver sheen, green, lizard skin, snowflake etc. Obsidian is metastable at the earth’s surface which means that over time the glass becomes fine-grained mineral crystals. The process can be sped up in the presence of water. This also means that Obsidian is only found in relatively modern areas, and in none older than the Cretaceous age (from 135 million to 63 million years ago). Although excellent quality obsidian specimens can sometimes be produced by surface lava flows, the best quality of Obsidian forms around volcanic vents just below the ground. As the lava squeezes in between rocks it forms layers of Obsidian that are relatively clear of impurities.
An important property of very viscous lava is that it does not mix well, which means that no obsidian from two different sources is identical. More so, each sample of obsidian can be traced to a specific volcanic eruption.