A few unique North American obsidian specimens to discover.

Dec 5, 2024
3 min read

Obsidian is a natural volcanic glass formed by the rapid cooling of felsic lava. This process results in an extrusive igneous rock that is amorphous and lacks a visible crystalline structure, which distinguishes obsidian from other types of magmatic rock. This rapid cooling prevents crystal growth and produces a hard, brittle material with very sharp edges, making it ideal for the manufacture of cutting tools.

Formation and Properties of Obsidian

Obsidian forms primarily in rhyolitic lava flows, where the lava is rich in silica. The high viscosity of this lava prevents atomic diffusion, thereby creating a glassy structure. It is composed primarily of SiO₂ (silicon dioxide), making up over 70% of its composition, which gives it its hardness and strength. However, this material is metastable on the Earth’s surface, meaning it slowly transforms into mineral crystals over time, especially when exposed to moisture—a phenomenon that accelerates the formation of perlite.

Although obsidian is generally dark in color, ranging from black to dark brown, its color can vary depending on the impurities present, particularly iron and other transition elements. Sometimes, inclusions of white cristobalite crystals or gas bubbles can produce fascinating visual effects, such as snowflake patterns (snowflake obsidian) or iridescent sheen (fire obsidian). There are also types of colored obsidian, such as rainbow obsidian, where iridescent streaks create spectacular plays of light.

Caption: Snowflake obsidian, Sevier Lake, Millard County, Utah, USA.

Obsidian Deposits in the United States

In the United States, obsidian is found in several volcanic regions, particularly in the calderas of active volcanoes. The most notable sites are found in national parks and volcanic areas in the West, such as the Newberry and Medicine Lake calderas in the Cascades. Other deposits also exist in the states of Utah, Arizona, New Mexico, Texas, and Idaho, as well as in Yellowstone.

The obsidian flows in these regions offer unique hiking terrain, and some of these sites have become popular destinations for collectors and geology enthusiasts. Obsidian can also be found in other parts of North America, such as Colorado, Washington, and even in the eastern United States, in Virginia, Pennsylvania, and North Carolina.

Obsidian in History and Ancient Cultures

Obsidian has been used by many civilizations throughout history to make tools, weapons, and jewelry. In Mesopotamia and in the pre-Columbian civilizations of the Americas, this stone was carved to create sharp blades, scalpels, and arrowheads. Obsidian played a crucial role in trade and exchange, particularly in societies where volcanoes were rare. For example, at Yaxchilán, an ancient Maya city, obsidian was used both to make tools and for symbolic rituals. The Maya and other Mesoamerican civilizations also used obsidian to make weapons such as the macuahuitl, a sword with obsidian blades capable of inflicting serious injuries.

Native Americans also played a major role in the obsidian trade, which was conducted across the entire continent. The different types of obsidian, produced by each volcano, allow archaeologists to trace the origins of obsidian artifacts and study the trade routes of these ancient civilizations.

Obsidian in Art and Jewelry

Obsidian, though fragile, remains a prized material in art and jewelry. It is particularly sought after for its deep colors and unique visual effects. Snowflake obsidian, with its white crystals forming a snowflake pattern, and rainbow obsidian, with its iridescent play of light, are particularly prized for creating cabochons, pendants, and decorative jewelry.