Agate is a micro-crystalline variety of quartz (silica), mainly chalcedony. 
Agate is known for its fineness of grain and brightness of color. 
Agates may be found in various kinds of rock, but mainly associated with volcanic rocks. 
The name Agate was given to this specimen by Theosophists, a Greek philosopher and naturalist, who discovered the stone along the shore line of the river Achates sometime between the 4th and 3rd centuries BC. 
Agate has a hardness grade of 7 on the Mohs' scale of minerals hardness.
It is widely used as cabochons in the jewelry industry. Moss agate is a semi-precious gemstone formed from silicon dioxide. 
It is a form of agate which includes minerals of a green color embedded in the chalcedony, forming filaments and other patterns suggestive of moss. 
The field is a clear or milky-white quartz, and the included minerals are mainly oxides of manganese which results in black color moss, or iron where the moss has a red color. 
Despite its name, moss agate does not contain organic matter and is usually formed from weathered volcanic rocks. Montana moss agate is an agate found in the Yellowstone and originally was formed as a result of volcanic activity.