Dendritic agate is a translucent, colorless to whitish-gray variety of chalcedony quartz, easily distinguished by its distinct tree- or fern-like markings known as 'dendrites', which are most often brown to black in color. It is considered to be one of the rarer and more sought-after varieties of agate stone, although it is technically not a 'true agate' since it lacks the typical banding of other agate stones.
'Dendrites', a Greek word for 'tree-like', are inclusions that appear organic due to their plant-like structure, but they are actually inorganic traces of iron or manganese. The iron and manganese ions gain access to the forming stone as weathering solutions from neighboring rock. The impurities crystallize inside the agate in branching formations seeping through fine surface fractures. In some cases, dendritic inclusions may form in spotted clusters rather than branching structures; these are sometimes referred to as 'mosquito stones'.