Well, they flex some when we go through the car wash and the jet water beats down on them, so I would guess that the glass is borosilicate for toughness. I'm sure if you throw a rock on them they would crack, and unless you drive through big tree branches that have to get pushed up over your roof, you would get possible scratches, We have used these same panels for years on our Boles Aero 28' travel trailer all over the roof, and have had branches, albeit thin ones, fall on the panels and they did fine.