Opal is a unique mineral, prized globally for its iridescent color play, which can be found in the rugged landscapes of Washington State. This mineral is a hydrated form of amorphous silica, essentially silicon dioxide with a water content ranging from three to twenty-one percent. The search for these gems, known as rockhounding, draws enthusiasts to the Pacific Northwest, where ancient volcanic activity set the stage for their formation. Washington’s geological history offers a compelling environment for finding both common and precious opal.
The Geology of Washington Opals
The presence of opal in Washington is directly linked to the Columbia River Basalt Group, which blanketed much of the landscape in lava flows millions of years ago. These basalt formations, particularly the Wanapum Basalt flows, were rich in the silica necessary for opal development. As the molten rock cooled, gas bubbles created voids or pockets within the basalt.
Groundwater circulating through the rock layers subsequently dissolved the silica, creating a silica-rich solution. This solution later precipitated as amorphous silica within the voids and cracks of the basalt, forming the opal. This process also led to the creation of opalized wood, where the silica solution replaced the organic structure of buried trees. While most finds are common opal, the conditions occasionally produce the gem-quality variety known as opal-CT.
Primary Opal Hunting Locations
The search for Washington opals focuses on the central and eastern parts of the state. The Saddle Mountains, a ridge rising above the Columbia River, are a well-known region for collecting, especially for specimens of opalized wood. This area offers open desert scenery where mineralized wood specimens can often be found on the surface.
Road cuts along State Route 17 in Grant County, particularly near Moses Coulee and Quincy, expose basalt seams that can yield common opal and opalized wood. Historically, the area around Whelan in Whitman County, near Pullman, was noted for some of the earliest reported precious opal finds. Moving toward the Cascade Mountains, locations like Chinook Pass and areas near the Greenwater River in King County produce different varieties, including fire and common opal associated with volcanic rocks.
Regulations for Collecting Minerals
Rockhounding in Washington requires collectors to understand and respect the regulations governing different land types. On lands managed by the Bureau of Land Management (BLM), casual collection is permitted for personal use without a fee or permit. The limit for collecting common minerals, including most types of opal, is set at 25 pounds per day plus one piece, with an annual limit of 250 pounds.
Collecting on US Forest Service (USFS) land permits the non-commercial gathering of reasonable amounts of material, though rules vary by district. Removing any mineral or rock is strictly prohibited in Washington State Parks, National Parks, and National Monuments, such as the Ginkgo Petrified Forest State Park. On private property, consent from the landowner is necessary before any collecting can take place. Collectors must use non-motorized hand tools and are required to fill in any holes they dig to minimize environmental impact.
Identifying and Extracting Opals
Successful opal hunting begins with identifying the correct geological environment, looking for exposed vesicular basalt or areas where the basalt has weathered away. Collectors focus on road cuts, stream beds, and exposed seams where the opal-bearing material may be visible. Basic tools like a rock hammer, a chisel, and a small shovel are necessary for safely extracting specimens from the host rock.
Identifying a true opal involves distinguishing it from other silica-based minerals like chalcedony or agate. Common opal, sometimes called potch, is opaque to translucent and lacks the characteristic flash of color. Precious opal displays “play of color,” caused by the uniform, microscopic structure of its silica spheres diffracting light. When wet, a freshly broken piece of opal exhibits a waxy or glassy luster, aiding in initial field identification.