Photo by Pat McMahan from http://www.agateswithinclusions.com/images/US/col/Del_Norte_Dendritic2.jpg

Dendritic agate has inclusions that look like thin, two-dimensional tree-like growths.  They are usually black or dark brown, especially if they are found in Montana.  Often dendrites form, between flat “waterline” bands of agate.  Dendrites may also occur in limestone, talc, sandstone, beryl, corundum, and other minerals.  Dendritic and moss agates both look amazing as cabochons, because they look like little pressed flowers or landscape scenes.  The specimen pictured is from Del Norte, Colorado.
Source: The Gemrock, 7/2014