Chert is a dense, microcrystalline sedimentary rock composed almost entirely of silicon dioxide. This rock was valued for its unique physical properties, allowing it to be shaped into sharp tools and weapons. Finding chert requires recognizing its distinct characteristics and knowing the specific geological settings where natural processes expose it to the surface. It occurs in a wide spectrum of colors and is a durable remnant of ancient marine environments.
Physical Characteristics for Identification
The most reliable sign of chert is its conchoidal fracture pattern. This fracture produces smooth, curved surfaces that resemble the interior of a seashell, often resulting in extremely sharp edges. A broken piece typically exhibits a waxy to dull luster, distinguishing it from the glassy shine of pure quartz.
Chert registers between 6.5 and 7 on the Mohs scale of hardness, meaning it cannot be scratched by a steel knife and can easily scratch glass. While impurities affect its color, chert commonly appears in shades of gray, brown, tan, black, or rusty red. The rock is fine-grained and dense, with individual crystals too small to be seen without magnification.
Geological Environments Where Chert Forms
Chert forms through silicification, where existing material is replaced with silica. Much of the chert has a biogenic origin, derived from the skeletal remains of microscopic marine organisms like diatoms and radiolarians. These silica-rich remains accumulate on the seafloor, where the silica dissolves and reprecipitates into nodules or layers within the soft sediment.
The formation process links chert to sedimentary strata, particularly within limestone and dolomite beds. Chert nodules are significantly harder and more chemically resistant than the surrounding carbonate rock. This difference in durability causes the softer host rock to weather away, leaving the chert exposed on the surface. Consequently, chert is often found as irregularly shaped nodules encased in a white or chalky outer layer, which is the remnant of the original carbonate rock.
Best Locations for Surface Collection
Chert is best found in environments where natural forces or human activity have disturbed and eroded the overlying soil and rock layers. Riverbeds and creek washes are prime locations because flowing water transports and concentrates the dense, weather-resistant chert fragments. Search along gravel bars and at the inside bends of streams where the current slows down, depositing heavier materials.
Road cuts and construction sites offer a direct look into the bedrock layers, sometimes exposing chert veins or nodules still embedded in their parent rock. These areas are valuable because they reveal fresh, unweathered material. Similarly, in agricultural areas, recently plowed fields can be excellent spots as the turning of the soil brings buried chert nodules up to the surface for easy collection.
Gravel pits and quarries, especially those operating in regions known for limestone deposits, expose large volumes of the chert-bearing rock layers. The mechanical crushing and sorting processes often separate the hard chert from the softer aggregate, creating piles of discarded material that can be sifted through. Focus the search on areas where the geological layers are visibly disturbed, revealing the durable fragments that resisted erosion.
Collection Practices and Legal Considerations
When collecting chert, it is important to bring a few simple tools, including sturdy gloves, a rock hammer for testing specimens, and safety glasses to protect your eyes when breaking rocks. Always be aware of your surroundings and exercise caution, especially near road cuts or in uneven terrain. Be mindful of sharp edges on freshly broken chert, which can be as keen as surgical steel.
Before removing any material, confirm the land ownership, as collection laws vary significantly depending on the location. Collecting small quantities of loose, surface rocks is generally allowed on Bureau of Land Management (BLM) and U.S. Forest Service (USFS) lands for personal, non-commercial use. However, National Parks, National Monuments, and State Parks strictly prohibit the removal of any natural resources, including rocks.
Collecting on private property always requires explicit permission from the landowner to avoid trespassing or theft charges. Collectors should embrace responsible practices by taking only a reasonable amount of material and leaving the area undisturbed for others to enjoy.