Moss agate is a semi-precious gemstone formed from silica. It is a form of agate which includes minerals of a green colour 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 or iron.

Despite its name, moss agate does not contain organic matter and is usually formed from weathered volcanic rocks.