The shape looks like a lightening bolt but I do think it's a crack in a pensieve. The shape on either sides of the cracks looks like a pillar - the same shape we've seen on both the UK and US covers.

To me, one looks young and the other looks old.

I've been thinking perhaps there's someone else with a scar like Harry's. We know, after all, that DD has a scar which is the map of the London Underground, on his leg. Could he have another? Or someone else ... *ponders*