“Dendrites are branched or tree-like inclusions, usually of manganese or iron oxides, that are caused by irreversible aggregation of small particles forming large clusters.
In other words, the metals in the dirt stain the rock. In this case Quartzite, in a random pattern (Chaos & fractal geometry) like water pressed between two panes of glass. This pattern is a stain that is formed when the seams are split open and the dendritic pattern becomes visible.
-from brochure by Zion Prospector Adventures LLC
I picked up a half dozen examples when I was in Tucson and expected them to be quick sellers on the rock show circuit. Sold one recently at Plantiques and this is the replacement one for that inventory. Only a few to be had so if you are in the area when Plantiques is open, stop on in.