Moss Agate is a bit of a misnomer as it neither has moss in it and also is not, in some cases, agate. Much of the material called Moss Agate is of the silicate family that has green dendritic inclusions that 'grew' within. Dendrite is Greek for tree and represents, in this case, the minerals that grow like plants as opposed to a crystal structure (like copper and silver if you have seen them raw). Such a mineral is growing inside the 'silicate soup' before it solidifies and the oxidation or valence of the mineral reflects the color of the 'moss'. For minerals turning all the other colors of the rainbow than green, dendritic agate is a more suitable term.
Moss agate can appear in many shades of green in clear, milky, or tinted agates. This piece has rich olive colored moss like pond algae and true agate banding in the center with a quartz druzy seam.
Just like the sand that leads to the pearl, inclusions can cause the silicates to form in grape like bubbles and wavy patterns. This moss agate shows the moss growing on the left side then being covered over by the layers of golden agate showing the growth terrain. Quartz crystals grew in last in the cavity creating this unique gemstone.
Moss Agate can be an incredible microcosm mimicking, at times, the very location that it came from. This piece has dark mossy inclusions, deep watery blue tinted agate, and quartz druzy at the top like a mountain or underwater scene.
Often the dendritic agate that I've worked with has creeping fronds of yellow ochre, oranges, reds, and even purple like Autumn leaves. The attendant agate around these inclusions often is a warm color of amber or pink leading to a lovely Fall like presentation.