The Pride of Yellowstone: A Guide to Montana Moss Agate

Montana agate is one of the more distinctive types of agate that’s emerged from the Americas and they remain a favorite of many rockhounds. This dendritic type of agate is famous for high translucency and beautiful inclusions, and it’s often worked into beautiful decorations.

What is Montana Moss Agate?

Montana Moss Agate (James St. John/cc)

Montana agate is a variety of included chalcedony found in… well, Montana. Like most varieties of agate, it’s generally banded. In this case, the bands are of an orange or yellow color and they generally maintain a high level of transparency despite the coloration.

Agate is, of course, a form of silica, specifically it’s a name used for included and/or banded chalcedony. It also forms into quartz and quite a few other stones. Specifically, it’s the name used for cryptocrystalline silica and it’s generally comprised of a complex intermixture of quartz and its polymorph moganite. The cryptocrystalline moniker means that ultra-thin slices and powerful microscopes are required to even begin to properly observe the crystal structure that makes up the stone.

The most common features in this agate are bands and dendritic inclusions. The latter are generally black and are included minerals which have crystallized into a tree-like structure inside of the chalcedony as it cooled. It may also contain black plumes and other features depending on the exact stone.

As a general rule, the most commonly included minerals for coloration in this beautiful stone are manganese oxides for the black coloration and iron oxides which give a wide range of red to orange colors depending on their concentration in the chalcedony.

Montana agate tends to be pretty solid, which makes it a favorite for lapidary use. The nodules of this material are much harder than the matrix they’re trapped in and they tend to weather out of the stone regularly while remaining fairly complete.

Montana agate is generally considered an alluvial agate, meaning that it’s generally found in deposits of gravel and soil after having weathered from the host rock. Fortunately for rockhounds, this generally means less hard rock breaking and more simple digging in order to find it in the areas where it’s found.

How Was Montana Moss Agate Formed?

The locations where Montana agate are found are near Yellowstone. Some of you are already nodding, as the hydrothermal origin of some forms of cryptocrystalline silica are well known.

The basic idea is that as the rock in the area formed, gas bubbles and other pockets in the stone emerged. While silica isn’t highly soluble in water, the high heat and intense pressure created deep under the earth can cause water and silica to form a gel. This is doubly true in areas with intense hydrothermal activity, of which Yellowstone has plenty.

It’s theorized that this silica-rich water formed into a heated gel that slowly filled in the holes. This may have come from rainwater or from underground hydrothermal activity, creating superheated water saturated with silica from the surrounding rocks and laced with the impurities that create the coloration of the stones.

Over time, this material cooled and the water slowly receded. The ultra-hot mixture separated into layers, creating the banding, and the minerals inside the silica gel crystallized, creating the dendritic inclusions.

And a few millions years later, we’re able to find it in the alluvial deposits of Montana, cut it, and turn it into amazing pieces of art!

While gas bubbles are often the origination of these cavities in rocks, it’s theorized that much of the Montana agate supply was actually formed in the cavities left behind from blasted trees that were buried in ash during an ancient eruption.

Where is it Found?

Montana agate is found in the Yellowstone river and surrounding gravel beds for the most part. The main thing that you need to do is make sure that you’re in an area where it’s legal to collect since Yellowstone Park itself isn’t a legal collecting area.

Fortunately, Montana is pretty cool with rockhounds and most areas Northeast of Yellowstone proper are open for collection. Indeed, in this state you don’t even need to have a permit to collect from National Forest land.

That doesn’t mean you’re free to run an excavator, of course. The usual restrictions apply, limiting you to hand tools and forbidding any operation that extensively disrupts the surrounding area. This is a great opportunity for rockhounds, so don’t mess it up for anyone else by doing extensive damage to area. Just being respectable is generally enough: don’t take excessive amounts of material, don’t dig into tree roots or other things which might damage the landscape, and fill in your holes. As long as you can manage that, and stay off private property, you’re free to collect your own samples.

Due to the land laws in Montana, the best place to find access that isn’t on private property is to go where fishing is allowed. A boat can be helpful. Anywhere under the high water mark is a legal location to hunt, but much of the land along the river is privately owned.

One of the best locations to look is on gravel bars in the river itself, and moving between them without a boat will be troublesome at best.

Tips for Finding Montana Moss Agate

As long as you’re in the right area, you’ll find that Montana agate is fairly easy to find. In this case, it’s actually important to pay attention to the time of year during which you begin your search however.

In general, spring before the floodwaters hit the river and in mid-June after they recede is the best time. Material in the river itself, as opposed to gravel pits along the river, is often of lower quality but more abundant since it’s been moved to this location. Most will be suitable for tumbles, at least, and larger, more complete nodules will generally be of better quality for lapidary use.

The upstream end of gravel bars will tend to contain more of these precious agates as well. It’s been my experience, hunting other agates in similar locations, that larger stones on the upstream end are also generally less damaged than those found further down.

Those found in gravel pits are often sun-bleached, so learning to recognize the shape is essential. Like most agate nodules they’ll either be potato shaped or a bit irregular in formation, and they’re easy to find when digging through gravel if you’ve spent some time in the field looking for them.

The material in gravel pits will often be less fracture prone since it’s exposed to less erosion than those actually in the river. The main problem is that they’re harder to recognize and that you’ll almost always end up needing permission fort these great spots to search.

Other types of petrified wood can also be found in both pits and gravel bars in the river. Keep an eye out for them, as some of it is very interesting material in its own right.

Tips for Working With Montana Agate

While I’ve been told that Montana agate isn’t all suitable for lapidary use, I’ve never found it to be a material prone to issues. Your milage may vary if you’re collecting your own material, as the stones I’ve worked were obtained through purchases or trades with other rockhounds.

Still, like any agate you’ll want to find solid material that doesn’t have a lot of surface fracturing. Some of the material isn’t exactly the prettiest either, and I’ve found far more of that in the stones I’ve slabbed than any real issues with internal fracturing.

In general, it works like most agates and the grinding process is fairly straightforward. Begin with an 80 or 120 grit wheel for the first pass, go up to 220, then 400. I’ll follow this with a 1200 grit wheel before going to a final polish with cerium oxide, although others have their own methods.

Montana agate is a favorite material of some lapidaries and it’s been covered extensively in literature. If you’re looking to find out from some people who actually make a living cutting the stuff, there’s actually a great book on it.

The How-To’s of Cabbing and Carving. Montana Agates and Other Chalcedonys is a great book for those serious about working with agates. The authors have worked more with this material than almost anyone else and their experience and information can be invaluable for those new to the lapidary arts.

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