Obsidian is natural glass created by rapid cooling of molten rock, or magma, associated with volcanoes. This "chilling" prevents crystallization of the minerals that make up most other rocks formed by volcanic activity. As a result, obsidian does not have a grainy appearance like other volcanic rocks such as basalt or granite; instead specimens look like chunks of man-made glass. Chemically, most samples of obsidian are similar to the overall chemistry of granite, containing mostly silicon dioxide that has not crystallized. Variation in the amounts of other elements in the glassy substance cause differences in color, though most samples fall within a range of colors from black to reddish-brown. All obsidian has a glassy lustre and, when broken, exhibits conchoidal fracture just like manufactured glass will.
Gemologists and rock collectors subdivide obsidian based on a sample's color and patterns of color. Single-color obsidian varieties include black, brown and mahogany, which has a reddish-brown color similar to the mineral jasper or the finished wood of the mahogany tree. The reddish color of Mahogany Obsidian generally comes from the inclusion of tiny specks of iron oxide minerals. Many samples of obsidian, especially those of the pure black Jet Obsidian, are translucent to transparent on a thin edge.
Perhaps the best-known varieties of obsidian, examples of which can be found at just about any souvenir stand or rock shop, are "Snowflake Obsidian" and "Apache Tears." Apache Tears are marble-sized chunks of obsidian, usually black but sometimes brownish. The size and shape of the obsidian pieces is natural in an authentic sample, usually resulting from tumbling in desert streams and polishing by wind action. Snowflake Obsidian is glassy rock, typically black, that's dotted with gray and white patches of crystalline material, usually cristobalite (an opal-like form of quartz). The cristobalite crystals often form a distinctive radiating pattern whose similarity to snowflakes gives rise to the name.
Most of the less common, and therefore more valuable, samples of obsidian display prominent bands of color. Such "banded obsidian” is believed by geologists to be caused by variations in the chemical composition of the magma as it squeezes through cracks and fissures in the earth like a sort of super-hot toothpaste. The names assigned to banded varieties of obsidian reflect the color combinations present.
Tiger Obsidian is a common variety of banded obsidian composed of black and reddish-brown or reddish-orange stripes. The black bands are the most common form of obsidian, which lacks mineral inclusions to give it color. The colored bands are much like mahogany obsidian, also containing microscopic crystals of iron minerals to give them their rusty color.
Other varieties of banded obsidian have been given fanciful names that indicate what colors and patterns are present. One such variety, Midnight Lace Obsidian, combines Jet Obsidian with other colored varieties (Mahogany, Silver, etc.). The term "lace" describes this variety's complex patterns of colored bands, which are believed to record stretching and rolling of the magma as it cools, not unlike kneading bread.
A few varieties of obsidian reflect light in a "sheen" pattern, much like light reflects off a thin layer of oil floating on the surface of a puddle of water. These reflective varieties include Sheen Obsidian, which gives the appearance of a golden sheen, and Rainbow Obsidian, which reflects colors in a rainbow-like pattern from a fresh, clean surface. Geologists believe that the presence of tiny bubbles of air trapped in the glass causes these sheens, while microscopic crystals of the feldspar minerals plagioclase and orthoclase control their color. Other sheen-bearing varieties include Silver Obsidian and Fire Obsidian.
Obsidian's glassy texture allows it to be worked like flint. Native Americans and other indigenous cultures have taken advantage of this property to use obsidian to make edged tools and weapons. Many cultures also prize obsidian, especially the more unusual and attractive varieties, assigning the minerals curative and healing powers depending on the color and appearance. It is also used for jewelry and small carvings.