Moss agate is a semi-precious stone with a marbled effect that resembles moss, trees, and landscapes. It's a variety of chalcedony, a microcrystalline quartz, and is technically not an agate because it's not banded. Appearance: Moss agate is known for its green shades, ranging from light and translucent to dark and vibrant. The stone's marbled effect comes from mineral inclusions like manganese and iron oxide, which create dendritic formations.