Moss agate is formed from silicon dioxide. The green patterns that this stone exhibits makes it look like moss. It is these patterns that attributed this form of Agate to be labeled Moss Agate.
Agate is ais a microcrystalline variety of quartz. Microcrystalline rocks are made up of many tiny crystals that can only be viewed with a microscope. Agate is yet another variety of Chalcedony
Chalcedony is a catch all term that includes many well known varieties of cryptocrystalline quartz gemstones. They are found in all 50 States, in many colors and color combinations, and in sedimentary, igneous, and metamorphic rocks. Chalcedony includes carnelian, sard, plasma, prase, bloodstone, onyx, sardonyx, chrysoprase, thundereggs, agate, flint, chert, jasper, petrified wood, and petrified dinosaur bone just to name a few of the better known varieties. (Credit: U.S. Geological Survey)
We know that Moss Agate can be found in Montana. It can also be found in South Dakota, Tennessee, Texas, Washington, and Wyoming along with many other varieties of agate that are found in these areas.