This unusual white stone often comes in clear or translucent pieces, other pieces are a more solid white. The truly distinctive characteristic of the dendritic opal are the small black lines or dendrites that mark the stone. The black lines within the rock are caused by the mineral manganese. These sometimes form unique patterns and scenes within the rock. Sometimes the dendritic opal is more opaque and looks less like an agate. Dendritic opal is very unusual. It is prized by rock cutters as no two pieces look exactly alike.