What you have there is a piece of igneous rock that shows darker mafic (iron /magnesium containing) and lighter felsic (quartz/feldspar) layers or sections that formed when (usually) the lighter colored rock intruded into cracks in the darker colored rock when the area that came from was formed (molten).
I have a couple pieces on my back deck as "weather rocks" The dark gets almost black when wet and I can tell on those iffy days if it is raining. Striped rocks are lucky. ;-)