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.
"According to Arabic traditions, moss agate bestows on the wearer the ability to differentiate between true and false friends. ...
Moss agate is stone which first became popular in recent times as a result of its healing powers. Due to its powerful effects on the islet cells of the pancreas, moss agate stimulates insulin production. By this mechanism the blood sugar level is adjusted and metabolic disorders such as diabetes are alleviated. ..." This according to: The Great Encyclopedia of Healing Stones, Fragrances & Herbs.
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.