Obsidian is a natural volcanic glass that forms when molten rock cools very quickly. This rapid solidification prevents atoms from arranging into an ordered crystalline structure, giving obsidian its distinctive glassy appearance. It is prized for both its beauty and its historical use in tool-making.
Understanding Obsidian
Obsidian is primarily composed of silicon dioxide (SiO2), typically 65 to 80 percent of its weight, similar to granite and rhyolite. The rapid cooling of silica-rich lava prevents mineral crystal growth, giving obsidian its characteristic glass-like texture. This lack of crystalline structure makes it an amorphous material, also known as a mineraloid.
While pure obsidian is usually dark, often jet-black, its color can vary significantly due to impurities or microscopic inclusions. For instance, iron and other transition elements can lead to dark brown or black hues. Small, white, radially clustered cristobalite crystals create the “snowflake” pattern. Tiny gas bubbles stretched along flow layers can produce a golden or silver sheen, while nanoinclusions of magnetite or hedenbergite can result in iridescent, rainbow-like effects.
Where to Find Obsidian
Obsidian forms when silica-rich, viscous felsic lava cools rapidly, often at the edges of a lava flow or volcanic dome. This rapid cooling occurs when lava comes into sudden contact with air or water.
Obsidian deposits are found in regions with active or dormant volcanoes worldwide. Notable areas include the Pacific Northwest of the United States (Oregon, California, Idaho, Washington, Wyoming), the Andes Mountains, the Mediterranean region (Iceland, Italy), Japan, New Zealand, and Mexico. Within these regions, obsidian is commonly discovered near volcanic vents, within lava flows, or transported into riverbeds and glacial tills.
Identifying Obsidian
Obsidian has several distinct physical characteristics. Its most prominent feature is a glassy luster, giving it a smooth, reflective surface similar to manufactured glass. Obsidian exhibits a “conchoidal” fracture, breaking with smooth, curved surfaces that resemble a seashell’s interior, often resulting in extremely sharp edges. This property made it highly valuable for ancient tool-making.
While typically dark (jet-black to dark gray or brown), obsidian can also appear red, green, yellow, orange, or blue due to impurities. It feels smooth to the touch, with a hardness of 5 to 6 on the Mohs scale, making it slightly harder than window glass. When differentiating obsidian from look-alikes like coal, basalt, or chert, focus on its uniform glassy texture and absence of visible crystal grains. Coal is lighter and softer, basalt is duller with a crystalline structure, and chert often has a waxy or duller luster compared to obsidian’s vitreous shine.
Safe and Responsible Collection
Collecting obsidian requires careful attention to safety. Essential gear includes sturdy gloves to protect against sharp edges, eye protection against chips, and appropriate footwear for uneven volcanic terrain. A rock hammer, chisels, and a durable collection bag or bucket are also useful.
Before collecting, understand land ownership and local regulations. Collecting on private property is prohibited without landowner permission. On public lands, rules vary by managing agency.
Bureau of Land Management (BLM) lands generally permit casual collection for personal, non-commercial use, with limits typically 25 pounds per day plus one piece, up to 250 pounds per year. Only hand tools are allowed, and collection is usually restricted in developed recreation sites or active mining claims. National Parks generally prohibit rock and mineral collection, with violations leading to penalties. State parks also have varying regulations, often requiring permits or prohibiting collection entirely; check with park authorities beforehand.
Practicing “Leave No Trace” principles is paramount when rockhounding. Minimize your impact by staying on established trails, disposing of waste properly, and leaving the natural environment undisturbed. Leave cultural artifacts, historical structures, and other natural objects as you find them, allowing others to experience the same sense of discovery. Responsible collection ensures the preservation of these geological wonders for future generations.