The grey streaks in this design are reminiscent of the veins that form in marble. As you may know, marble is a metamorphic rock. It’s a heated, squeezed, and recrystallized version of limestone. The grey bands in marble started out as layers of clay. When a rock undergoes metamorphism, it gets compressed, buried deeply, and pushed, pulled, and twisted. That’s what gives marble the flowing veins of color.