Moss and tree agates are formed from chalcedony, though this stone lacks the characteristic banding that would make it a true agate. The green inclusions are iron or manganese, and often look like ferns or other kinds of vegetation.
According to The Crystal Bible by Judy Hall, moss agate is "a stabilizing stone strongly connected with nature" and is refreshing and a highly optimistic stone. We have a customer that swears by putting pieces of moss agate with her potted plants, and we find the stone to be very linked to the vitality of life.