It comes from near the Yellowstone River.
Montana Agate is one of the alluvial agates, it is not found in-site, but in the Flaxville gravel deposits scattered over a huge area encompassing thousands of square miles. Montana Moss Agate is found throughout the 670 mile length of the river.
The agate originally was formed in the Yellowstone National Park area of Wyoming as a result of volcanic activity approximately 65 million years ago.
In Montana Agate cabochons the red color is the result of iron oxide, the black and brown is the result of manganese oxide and the red and orange came from iron oxide. On very rare occasions you can get a green from copper.
It is considered a moss agate because of the mossy shape of its inclusions, but more often it is banded, polka dotted or has dendrites.
Most of the Montana Agate rough is broken up and found in smaller pieces. Chunks larger than 10 pounds are rare. However the largest piece ever found weighed 63.5 pounds!
Montana Agate generally has a coating or skin.
Agate is a microcrystalline Quartz and has a hardness of 7, which makes the Montana Agate designer cabochons very wearable.
Montana Agate has been the state of Montana's official state gemstone (along with Montana Sapphire) since 1969.