This is a very common type of rock chip. Notice the point of impact in the center of the damage on the left side. You can see the dark area which extends out from the damage point on each side of the bulls eye. When the windshield is damaged, air, dirt and debris are forced into the break causing it to appear darker in color. The repair process consists of alternating pressure and vacuum cycles. The vacuum cycle draws out the trapped debris, air and moisture while the pressure cycle injects resin into the damaged area eventually filling it. The “after” photo shows little trace of the original damage, leaving a small, faint gray dot at the original impact point.