Obsidian is a silica rich volcanic glass.

Chemical Formula: n/a
Class: Igneous glass
Hardness: 5-5.5
Coloration: Black, brown, red

How to identify it/Unique Characteristics or Properties: Obsidian is a silica rich volcanic glass. It forms generally from magma flows that would form rhyolite. If that magma flow loses its’ water content, it becomes thick and crystals do not grow in it at all. Without crystals, what is left is volcanic glass. This is why obsidian is less a chemical formula and more of a process, almost any glass that is formed naturally through magma/lava is called obsidian. This natural glass is slightly stronger than our normal glass, and can be a variety of colors due to different impurities in it. In fact, obsidian is unique to each flow that produces it, and is sometime very distinctive with colored bands and sheen found only in specific lava flows. This has allowed geologists to track ancient trade in obsidian halfway across continents from its source.

Obsidian breaks in conchoidal fractures, basically it doesn’t break in straight edges but in curves so it can make very sharp edges. This has made obsidian a natural for making extremely sharp edges in tools and weapons. In fact, the lack of crystals makes obsidian able to hold an edge even sharper than steel (whose hardness comes from crystals in it). However, from a geologic perspective obsidian doesn’t last long and can break down relatively quickly into other minerals when water works its way into the obsidian and changes its silica base. When Obsidian breaks down into perlite, it slowly chips away into small perlite beads as the water works its way deeper into the piece of obsidian, eventually leaving only small teardrop shaped pieces of obsidian, commonly called “apache tears” (see picture below of one in process and one of just the “tear” left). When obsidian sometimes has crystals of perlite of christobalite in it, (either during the cooling process or as the early stage of it degrading from water exposure) often called “snowflake obsidian” (see picture below). Obsidian can also form long spars from the lava dripping into piles of ash, making “obsidian needles” like the picture above.

Where to find it in…

The Northwest: Glass Butte, Oregon is an entire mountain made entirely of obsidian.
Other: Widespread around volcanic areas. Iceland, Italy, Japan, Mexico.
Past Uses: Obsidian was used extensively in native american life as knives, tools, and arrowheads. Today, obsidian is used sometimes in surgery due to its sharp edge. obsidian also once was carved into objects as it can be polished into a smooth, shining, surface.
How it was Made: See general description above. Alternative methods is local silica sand melting from the heat of volcanic activity.
Interesting Facts: Native American arrowheads from one location have been traced to obsidian formed over 700 miles away.
More Information on the Web: Wikipedia: Obsidian Make your own Arrowheads