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Hunting For Montana Agate

Posted by Terry Diede on

After resolving some health issues, the natural “hunting and gathering” instincts kicked into full gear and we were able to enjoy agate collecting for the first time in many years.  A trip to visit family in Eastern Montana gave us the perfect opportunity to explore and acquire some exceptional Montana Moss Agate.

Montana agate is noted for the exceptional clarity of matrix, highlighted with contrasting red/orange bands of color sprinkled with black mossy dendrites or trees.  In addition, some of the bright colored contrasting patterns form uniquely beautiful patterns.  If one is really lucky, a rare thin slab will produce an “iris” agate revealing a rainbow of colors when back lit or shown in sunlight.  An array of precisely spaced deposit lines causes this diffraction grating effect.

These agate nodules are relics of ancient geological activity in the Yellowstone Formation eons ago. The Yellowstone cauldron has erupted multiple times filling voids within the basalt and ash beds with hydrothermal deposited silica, colored by trace amounts of iron and manganese.  Being harder than the surrounding matrix, as the formation eroded the agate nodules tumbled out and were washed down local streams and riverbeds finally settling in the plains of eastern Montana.  These deposits are now referred to as the Flaxville gravels.


One of the countries last free flowing rivers, The Yellowstone has offered up agate for both practical and artistic use.  Native Americans first valued and used these rocks for knapping arrowheads and spear points. The lapidary community has enjoyed them for crafting for more than one hundred years. 

After a record year of flooding the Yellowstone River churned up a new supply of high-quality agate. Local rockhounds and entrepreneurs brought to market some of this harvest which we were lucky enough acquire.  Additionally, we were able to spend several days walking the gravel hills and fields of family and neighboring farmers.



Field agates typically are covered in a crusty white patina that forms with years of exposure to the elements.  River agate exhibits a dark “orange peel” texture on the outside surface.  All told, we were able to bring home a great selection of slabs, river sourced and field agate which we will be cutting shortly and making available for purchase.

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