Montana Moss Agate Location

Rockhounds from all over the world come to Montana in search of Montana Moss Agates. The Montana Moss Agate is found in gravels of the Yellowstone River Valley and its tributaries.

The Montana Moss Agate gets its name from black (manganese) or reddish-brown (iron) dendritic structures found within the agates. These dendritic structures (multi-branching, tree- or moss-like) are what make the Montana moss agates special and unique in the world of agates. Classic agate characteristics such as banding, plume structures, tube or eye formations, and visible quartz crystallization are also found in Montana moss agates. The rare prismatic light scattering effect, iris agate, is also observed in some Montana moss agates. The background color of Montana moss agates is often clear to slightly cloudy, or even bluish.

  

Agates can be found in Yellowstone River Valley gravels from north of Yellowstone National Park to the North Dakota border. Some experts suggest that the best stretch for hunting classic Montana moss agates is between Custer and Sidney (Harmon, 2000). Most people find agates in the gravel bars along the active Yellowstone River channel. There are numerous public fishing and recreational access sites along the river. Right-of-way corridors at bridge crossings are also potential public access points to the river.

Once on the river, Montana’s stream access law allows for anyone to walk or boat anywhere on the river, as long as they remain below the high-water mark. Agates can also be found in the “terraces” above the active river channel. Rockhounds have found many agates in mined gravel pits in these terrace deposits. These terrace gravels can be found tens of miles away from and 1,000 feet in elevation above the nearest active river channel, indicating the vast meanderings of the river throughout time. The tributary drainages to the Yellowstone often contain agates that likely eroded from these terrace deposits. Much of the land with terrace gravels is privately held, so make sure to ask permission before agate hunting on private land. 

The outward appearance of a Montana moss agate can often hide the beauty within. The outer surface of an unbroken agate can appear drab brown or black to the casual observer. The surface of these drab-colored rocks may be covered with small (<1 cm in diameter) semicircular conchoidal fractures. Other agates may have irregular surfaces and can be quite pitted. Some agates will have a prominent interior band of quartz crystals that is lighter than the rest of the rock. Agates that have been exposed to the sun for a long period of time develop a distinctive, highly visible white crust. This white crust is often the easiest way to spot an agate in undisturbed terrace gravels. Although the banding or moss is readily apparent in many specimens, some agates are difficult to identify until they are wet. For this reason, serious agate hunters carry a spray bottle to wet their finds before lugging them home. 

   

Tom Harmon, and Jeff* and Jay Nesper have wonderful books and articles dedicated to Montana Moss Agates. Also, many YouTube channels are devoted to finding agates along the Yellowstone River.

Check out this great article on places to rockhound for Montana Moss Agates

 

*A special thank you to Jeff Nesper for sharing his time, knowledge, and pictures.