Montana Moss Agate is one of the alluvial agates. Found not in-site, but in the Flaxville gravel deposits scattered over a large area encompassing hundreds of square miles. The beauty of this is that they cannot be claimed, mined and dug-out by a few enterprises but will be available, in smaller numbers, to public and collectors for tens and hundreds of years to come. The fresh exposures do get hunted quite diligently each year, but new agates are always found by the persistent collectors.
Montana Moss Agate, a name given to the beautiful chalcedony found, most abundantly, in the alluvial gravels of the Yellowstone River, would probably be better named Yellowstone Agate, because it's genesis was centered in the Yellowstone Park area. The actual tremendous volcanic activity that produced the conditions necessary for the formation of agate, spanned hundreds of miles and millions of years.
Although it's genesis centered in the Yellowstone park area of Montana and Wyoming, this volcanic activity ranged from the eastern Rocky Mountain front in south-central Wyoming to the western front of the Black Hills and north across eastern Montana and into Saskatchewan and Manitoba Canada.
Eastern Montana was mostly a shallow inland ocean, almost a swamp with huge forests lining it's shores and islands of volcanoes spewing forth lava to entomb parts of the forest in lava and ash. The bowels of the Yellowstone bulged and roared and flowed mountains of lava that decimated thousands of acres of mighty redwoods and sequoias for hundreds of miles around. This decimation continued for hundreds of years with layer upon layer of forests growing up and then being driven down under the ponderous weight of all the mega-tons of lava and ash. The hot lava devoured most of the wood in it's rush to cover the trees, but some of the shape and ingredients of the limbs remained trapped in the cooling lava. When the time of volcanoes and lava was subdued and the rains came, mineral laden silica-water flowed into the cavities and pockets left by the dying trees and bubbling lava. As flow after flow slowly filled the pockets with liquefied silica, Montana Agate was born.
General Agate Information:
Agate is one of the most varied and desired forms of "chalcedony" which is one of the many varieties of quartz. Scientifically classified as a crypto-crystalline or micro-crystalline quartz, it has a hardness of 7.
Quartz, in all its forms, is the single most abundant mineral on earth, making up almost 12% of the earth's crust.
Quartz varieties are separated into two basic groups, macro-crystalline and micro-crystalline. In macro-crystalline quartz the individual quartz crystals can be seen with the naked eye. In micro-crystalline, sometimes called crypt-ocrystallline, the individual crystals are to small to be seen even under slight magnification. Agates and chalcedony in other forms, like chrysoprase and carnelian, jaspers and flints, are some of the crypto-crystalline forms of quartz.
Some examples of the macro-crystalline forms of quartz are amethyst, ametrine, citrine, rose quartz, rutilated and smoky quartz.
Agates in many different varieties are distributed worldwide but localities of agate beds of major significance only number less than a hundred.
Humanity has admired agates for thousands of years. In ancient times the beauty and durability of agate prompted them to use it in both practical and ornamental forms. It was believed that agate has unique properties that protected the wearer from dangers and promoted strength and healing.
Agates in general come in many different forms and are formed in at least five different ways. The main conditions necessary for agate formation, are the presence of silica from devitrified volcanic ash, water from rainfall or ground sources, and manganese, iron and other mineral oxides that form bands and inclusions.
There are some good things being written about complex agate formation and one of the best I have found is "Banded Agates" Origins and Inclusion by Roger K. Pabian and Andrejs Zarins, Educational Circular No. 12.
A large portion of the agates found around
the world are found "in-site" where they were formed. The main problem
with this is that once the deposit is dug out, they are gone. Unless a new
deposit of a particular agate is found in the area, they are gone forever.