Moss agate is a semi-precious gemstone formed from silicon dioxide. It is a form of chalcedony which includes minerals of a green color embedded in the stone, forming filaments and other patterns suggestive of moss.
In the U.S., Montana moss agate is found in the alluvial gravels of the Yellowstone River and its tributaries between Sidney and Billings, Montana. It was originally formed in the Yellowstone National Park area of Wyoming as a result of volcanic activity. In Montana, moss agate's red color is the result of iron oxide and the black color is the result of manganese oxide.